Joy... of Building and Driving

[spannermanager]

Hi Dave, yes, i ran 300 lb rears in a successful rally-cross 1303, i had to run double shocks to control those consistently, it was heavier on the rear than yours would be too, being water cooled and 5 speed 4w/d trans system, it would pick up mud as added weight also, but i see you do plenty of that also lol, maybe your shocks are fading out....
i see your springs are only 8", maybe the suspension range is too short, the secondary behavior on depressions is typical unloading forces, either too slow on high speed re bound damping or, as you suggested, bottoming out and rapid unloading, maybe its going coil bound?. whatever, those spring rates require some careful damper matching, i would try more rebound damping before going up on spring rate, you must be nearing the point of loosing traction to a stiff set up option, mine broke my neck away from the lights with traction, and i had softer springs for the rain, but they became redundant as the added mud dragged the ride height down too much, but they worked well on wet tarmac circuits with no mud. just a few of my thoughts and findings for you, keep up racing it often mate....