As others have said, there is something wrong with your setup. You should fix this before you begin experimenting with add-ons like a rear diffuser. Are you sure you are feeling lightness in the front end and not bad tracking/tramlining? This can be pretty disconcerting at speed. Assuming that is the problem, in addition to checking the alignment settings, also check your front & rear wheel bearings for excess play. As for alignment, below are the Westfield factory settings and what I am running on my car (SEiW with widetrack, no ARBs). The latter settings came from some racers in the UK who have experimented a lot both on and off track and they have transformed the handling of my car. They are a little aggressive for the street and do have some wear problems in the front, but the car feels far more planted, with much better turn in than stock. A few Westfield-specific things to consider:
Front ride height should be set so the imaginary line between the inner and outer pivot points of the lower control arm is parallel to the ground
Rake is critical. The more you stray from the recommended range of 15-20mm higher in the rear, the more problems you’ll have. The factory points for these measurements are in the build manual. Let me know if you don’t have this.
Factory (Front/Rear):
Camber: -0.25 to -0.5/-0.25
Toe: 10 min toe in/30 min toe in
Rake (Rear-Front): 14mm
My settings
Camber: -2.5/-1.5
Toe: 15 min toe out/30 min toe in
Rake: 20mm
-John