You can leave the 54T in it if you wan to, I don't know if AE has a tall enough plastic to match that. The steel gear will work fine, just the plastic one will b quieter and be a somewhat sacrificial gear if needed.
This chart will help select the proper pinion for your application,
http://scriptasylum.com/rc_speed/_top_speed.html
Gear 40mph tops and it should stay nice and cool.
I'm sure some fellas who also own the RC8T will add to this, Enjoy !