Why don't rc cars use rack and pinion type steering? I'm a layman but I think it could reduce space, simplify and reduce the number of components. The servo could directly drive the rack by putting a pinion on it. The awesomatix touring car already seems to have a well designed rack seen here: http://awesomatix.com/steeringsystem.html
but the drivetrain would have to be belt-drive. Perhaps a belt on top but geared at the differentials like an SC10 4x4. This would allow centering of half height steering servo on the chassis. Basically center everything.