You could go up to 8"... after that, you'll have camber issues. You need to be able to drop the Beam pivot points a little lower. I know some of the skyjacker kits have extra holes in the drop brackets that would accommodate the next size lift as well.
If you can indeed drop the beams more, you could run the 8" coils and the 2" spacer. At that point an Extreme drop pitman arm most likely won't work, and you'll need to make a steering setup similair to the one that superlift offers... At that point as well, even with extended rad arms, you may want to drop the arms 2" to fix caster issues.
This is going to be one of those things where you're going to need to tinker a bit to make it streetable.