The bumpstops would only need to be shortened if the car has been significantly lowered, which I doubt a set of springs would do on these cars. Your problem is simply that, well, you changed the springs.
Edit:
To elaborate, if the car is slamming into bumps and dips as if it has no suspension at all, you have three possibilities:
1. Coil-bind. The spring is being 100% compressed when you smack bumps.
2. Bottoming out the shock. The shock is at 100% compression. Dangerous.
3. Lowered car is sitting on the bump stops, which aren't nearly as progressive/forgiving as springs.
All three point to faulty design.
Threaded Mode
