ironically enough, more then 1" and you are already changing the geometry of both the suspension and steering rack.... so you have already broken the barrier, just cut them to whatever you want now as it wont get much worse.
on a side note the reason you probably "feel" like the center of gravity is out is because you cut the springs and now have really nothing working on keeping the car upright. when lowering i've recomended my HR race springs, lowers just a tad, but its a stiff progressive spring... lamens terms: it makes handeling beter then the other springs, period.
SS are nice, but too low of a drop for me, at that point you will bottom out and not be able to grip the corners as well due to the low travel area of the suspension with the short springs.... but hey, take it for what its worth.
Torinalth