You sure about what you got? According to my sources the only TRX450 available in 2006 was an "R" not and "ER". Give me the VIN and I can verify what you have.
Anyway, for more power in the take off and up hills, gear down. Go down one tooth on the front sprocket or up about 3 on the rear sprocket. Or - you might go to a smaller OD rear tire. You can get a bit of a boost by changing the exhaust, but if you are going to change the exhaust, you should go the whole route and make changes to the intake. Then have the carb rejetted. Since you live in CA (and I believe Victorville is in the "high desert") you might be jetted too rich for the altitude. Going down about 2 sizes on the main jet could improve throttle response and power. But if you put on a different exhaust and reduce intake restriction, then you might not need to change the jetting.
I live in Illinois, so what works here might not work there, but the gearing works everywhere.