If you're putting all the power down on a 9000kV motor, you WILL have to slow down for corners. That is crazy power!
Foams won't help you, they will make you slide more at that sort of power.
I doubt ANY 1/10 set-up would allow you to turn well at full power.
For the servo, if the steering is lagging or not centering after a turn, you need more torque. Make sure it's metal geared and ball bearing at the least for the strain you will be putting on it