I've had the same problem recently on my C4.1... I dont think its anything to do with the cars built in servo saver or spring as its only really on full lock. The problem is that the servo arm isn't long enough to put sufficient pressure on the steering when on full lock (in particular when turning left).
I just changed the servo arm for a longer one (I used a fastrax one) which has cured the problem... so much more sterring, definitely getting full lock and the inside wheels doesn't 'twitch' when going round a corner on full lock.
http://www.apexmodels.com/gbu0-prodshow/FAST311B.html
It allows you to keep the standard spring in the servo saver so that you're not damaging your servo. Looks pimp too
Hope this helps