if ur gonna get a rrp steel gear set or somthing steel, put it in a beedblaster to get the week anodizing off (if it is anodized) then spray can it black, mine started rusting like crazy til i painted it.
I am not sure about the spur gear but the slipper is way different on the old 2.5 maxx then it is on the new 3.3 maxx, the 2.5 maxx use like 12 little rubber type plugs that go into the holes on the spur gear, if you want to make it better get one of these--> http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXBJB6&P=7 and here is some bodies http://www3.towerhobbies.com/cgi-bin/wti0095p?FVPROFIL=++&FVSEARCH=t+maxx+body also a super charger will not help you with getting more power, as for your air intake mod it seems like it would not hurt any thing but I don't think it will help, to it seem like it is too long of a run for the motor to be pulling air through befor it gets to the motor, but that is just my opinion. for cvd's i would recomend getting MIP's I think these are the ones for your t maxx http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXUR79&P=7