they were the lowest spring that is progressive. I was worred about bottoming out in the rear with a fixed rate spring.
the rubbing issues with 10.5's is more exhaust related than lower related if you have th right offset. some after market exhaust kits can have some issues from what I ahve read. deadlurker had some issues with his bassani system if memory servs correct.
I only have 10" rims but I have no issues what so ever.
one thing to keep in mind is that you have a mach1, not a GT... you car is a bit lower from the factory. so the drop will not be the same as what most people have stated. just something to think about.... unless they are advertised X drop on a Mach1