Well the performance is gonna cost heat.But there's not going to be huge differences in the 2 motors.Either will get hot and it will be up to you to control that.

i will keep the 7xl

Go with the 8XL...the 7 pulls to many amps and will puff up your battery (I know from experience) and the 9xl might not be fast enough to impress all the speed demens out there...so the 8XL is the only logical choice (for 4s).

O yea one more thing there is a very big difference between a 7xl and an 8xl the 7 is 2400kv and the 8 is 2100kv...the 7 runs way way to hot (around 195 deg) hot enough to demagnetize your motor and trash your pack...ask mike cronin what he would do...he will tell you to get the 8xl!