This is also called a "pushing" condition, on full scale racers.
(Understeer = pushing = nose of car wants to hit the wall.
Oversteer = loose = rear end of car wants to hit the wall.)

If it's truly understeer, it won't matter which servo you use.
It simply means that the car wants to go straight ahead, even though the
wheels are turned plenty, and it should be turning. The problem with a lot of
RC cars is that they're at the very brink of lifting the front wheels off the ground
under hard acceleration, and this means you have less steering authority, as the
front tires just skitter along the ground, instead of biting and making the turn.

Positive traction rears, or very tight diffs, approaching posi-trac, make it even worse.

You'll have to transfer some weight to the fronts to alleviate, whether that be
through adding ballast, or adjusting springs and shocks or whatever.
It's all about having a full width patch of rubber firmly planted on the ground.
Sometimes camber adjustments will help out with this situation, too.
(If you have too small a contact patch while under hard acceleration)
You may make adjustments (or put lighter oil) in the diff, to loosen the car up some,
but I guess that depends on your setup a lot. If you have a locked-in rear, you're stuck.
If you feel that the servo isn't strong enough to turn the wheels while you're
really hauling, then yes, by all means try a better servo. It's just not that easy most times.


Also, just like in the full size cars... sometimes you simply have to slow down to
properly make it through the turn. Sometimes you have to go slower, to go faster


Hope that gives some ideas... and do know that I speak more from full-size than
from RC experience, but most of the stuff applies... I still have a slight problem with
my own RC10GT, but I blew the engine, so I've not had much of a chance to tweak.