I have the 6" springs in the back of my Burb without the spacer but I still have the Autoride functional and the air bladders inside the springs.
Without the bladders and before I adjusted my Autoride links it sat lower but would bottom out over bumps. This was WITH the free travel mod.
I ended up cutting a notch in the crossmember between the frame rails to give the panhard bar bracket on the axle a little more room (you could see marks on the crossmember where it had been making contact). I also trimmed the "ears" on the panhard bar bracket so they didn't stick up past the bar itself.
Here is a pic of the area as it was from the factory...
You can see the squared off "ears" of the bracket on the axle and where they had been making contact with the crossmember above.
This is after I notched the crossmember...
And this is after I trimmed and rounded, and essentially got rid of the "ears" on the axle bracket...
I have the Spohn adjustable panhard bar... not sure if you could trim that much with the stock bar in place or not.
As far as going lower than the 6" springs... I think your ride quality will suffer tremendously, not from the coils being cut... but from the axle hitting the frame more frequently. I'm not sure there are many options other than cutting the coils or heating them (which is worse, IMO) without raising the upper spring perch which would take a ton of work (lifting the body off the frame).
Either way... I hope these pics help you out.