Anyone who plays Rollcage, or GRIP, with a keyboard is deliberately handicapping themselves (sorry). Rollcage and GRIP both have high speeds, high velocities, that unavoidably require a good level of control. The binary nature of a keyboard simply doesn't provide this, and using an analog controller will always work better.
So, while we will of course support keyboards for GRIP, be aware, that player demands for high speed, will make it difficult to play by comparison. If you're serious about gaming, you'll get a controller.
With regard to air time, and getting your jumps done right, I think Broscar nailed that pretty well - landing won't be your problem most of the time - but smacking into a side-wall will be. Air time will help little in these cases.
I did mention driving aids in another thread however, we were could have automatic correction of the car in the air to assist in landing should you get past those pesky walls. Then it's up to you whether you want this additional assistance or not, or whether you want to go hard-core.