I had the same problem. There is a setting in the .PLR file for the active profile named "Steer Ratio Speed", which by default is set to 25.0 (I think). If you set this to 0.0 instead, steering will not be affected by speed. I just found the setting and it seems to solve the problem, but I haven't yet had time to do much driving with it.
(Sorry for bumping an old thread, but it sits high up in the Google results, so I thought it helpful to post a solution here)