The stock steering servo and servo saver are really limiting as well.
While you might have a centering issue, either with trims or the servo horn as mentioned already, I really didn't get some good steering out of my S-25 Savy until I upgraded to a high-torque metal gear steering servo, cam-type servo saver from HPI, and a hump pack for power instead of the AA batteries.
I've had my S-25 for six years or so. Any other questions about 'em fire away.
HTH...
Mark