If you are actually running races, you would probably benefit by installing a stage 2 Hot Cam. You will lose a little bottom end, but it will liven up the mid range to top end. A stage 2 utilizes stock valve springs and auto decompressor and provides excellent throttle response.
Personally, I would run with the complete stock air box and use either UNI or TWIN AIR air filters. I use UNI filters and have two. One in the machine and one ready to go in. I have only changed a air filter once during a race. It was a 2 hour scramble race and conditions were extremely dry and dusty. Only run with the top off the air box if your races do not have any water crossings. If you opt to use a K&N filter, get two (one to run on and one prepping to run on). It takes several hours to properly clean and oil a K&N filter and it must be done after every race if conditions are dusty enough to get "race face" (clean around the eyes where the goggles have been and dirty around the nose and mouth. You know - blow mud out of your nose for a day after the race? The cam does not affect jetting a great deal.
What novice tuners think is usually "more fuel equals more power". In actuality, less fuel means more power.
Power comes from the heat of the burn. A rich mixture is easier to ignite, but burns cooler. The phrase around drag racers is "lean is mean!" A lean mixture burns hotter and generates more power, but if you get too hot you can do engine damage and you want to avoid that. I always recommend that you make the changes you want to make and then run the engine to see what if any changes are going to have to be made to the jetting.
You jet the carb for the characteristics of the carb, not the engine. In production, the carbs are put on a flow bench, air is forced through the carb by vacuum and sensors measure the ratio of fuel to air at various throttle settings. The air flow volume is determined by turning an engine (complete with exhaust and air filtration system in place) of the design the carb is being selected to control with an electric motor at various RPM's and measuring the flow at each RPM in order to duplicate the condition on the flow bench. The selected carburetor is then calibrated for the desired air/fuel ratio at each RPM on the chart.
Therefore, changes to the exhaust (pressure side of the engine) affect jetting less than changes to the intake (vacuum) side of the engine. Outside the assembly facility, jetting becomes a "seat of the pants" operation.
Make your desired changes (except camshaft), start the engine, bring it to full operating temp (run the engine till the cooling fan cycles on then put a window fan in front of the vehicle) - at this point you are set to adjust the idle air/fuel mixture - adjust the fuel screw for the smoothest idle (no more than 6 full turns from lightly seated) (typically the fuel screw will be about 4 full turns from the factory) - if you have to back the screw more than 6 full turns out (out is richer) (BTW, you will need a special tool to adjust the screw unless you replace the screw with one you can turn by hand - my favorite is the fuel screw with a flexible cable), then you need to increase the pilot jet one size. With the idle set, you are ready to adjust the mid-range.
To set the mid-range, take the quad out where you can run it in fourth or fifth gear at a steady throttle. You will want to run at a constant 1/3 to 2/3 throttle setting for at least 300 feet - the engine should run without surge (lean) and without blubbering (rich). If lean, you want to lower the clip on the slide needle (raise the needle) and if rich you raise the clip (lower the needle). Usually moving the clip 1 notch is sufficient to attain the change necessary for proper running. If you have to change the needle clip more than one notch, first verify proper float level. If the float level is acceptable, then consider changing the slide needle for a sharper or shallower taper depending on the required air/fuel ratio change desired.
Finally, take the quad out and stab the throttle wide open. You want clean, linear acceleration all the way to the rev limiter. If you experience a "bog" (the hardest thing to get rid of), bog is usually caused by all air and no fuel. This is where the accelerator pump becomes a factor. There is a "leak" jet in the bottom of the float bowl that adjusts the amount of pressure and quantity of fuel the accelerator pump delivers into the venturi of the carb upon throttle opening. To little fuel and the engine stumbles - too much fuel and it is a waste of fuel and potential fouled spark plugs. Honda does not offer optional jets for the 450ER, but other models using the same style carb does have either larger or smaller jets. I worked on a Yamaha that the stumble was so severe, the engine would simply die when trying to accelerate out of a corner. If you eased into the throttle it was fine, just don't try to accelerate with the throttle wide open. I installed the smallest leak jet offered by Yamaha and it still wanted to die. I installed a leak jet for a Honda CRF250 which was smaller than the smallest offered by Yamaha. It was better, but still unacceptable. I shortened a phillips screw with the same thread and closed the hole. The problem was solved. It has been the only carb that I had to resort to blocking the hole. All others that needed an adjustment were corrected by changing leak jet sizes.
If it will not accelerate to the rev limiter cleanly, change the main jet - if it tapers off before it gets to the rev limiter like it wants to do more but can't - jet up. If it revs to a point and just quits, blubbers like the choke is on, just hits a certain rpm and flat won't do any more, sometimes if you back off the throttle slowly it will start to accelerate again, but there is a point that it just won't get past - it's too rich, jet down 2 sizes at a time till it runs right.
The carbs are not too hard to work on, but getting them off is sometimes a PITA. My CRF250 requires removing the seat, fuel tank, muffler, sub-frame (with air box) and disconnecting the top of the rear shock. My TRX400EX only requires removing the seat and loosening the air box, but I usually just remove it for ease of access. The carb on the CRF is near identical to the carb on your 450ER while the carb on the 400EX is more similar to the carb on the XR400R motorcycle. My CRF came with a special tool for extracting the slide needle. While not necessary, it is a handy tool. Basically just a piece of stiff wire with a piece of vacuum hose glued onto the end of it. You just push it on the needle and pull the needle out. I use it for putting the needle back in, but it's a little tricky. You just barely put the needle into the hose, insert it and wobble the tool till it comes off the needle leaving the needle in place.
Once you have done all this is and are satisfied with the way the engine runs, if you want, now change the cam. There is a break-in procedure for the cam that is important. If you install a new cam, get new valve lifters for the intake valves. Set the valve clearance as normal increasing the intake clearance by .002
Hope I covered everything