Finally got new wheels and tires installed.
Tires: Cooper Discover ATP, E Rated, 265/70/17
Wheels: 18x8.5 MB Razors.
All from DiscountTireDirect.com, mounted/balanced/delivered/tax free.
Trust what other have said...This is THE LARGEST tire that will fit...and it will require trimming.
I installed the tires without any modifications, and there was
rubbing on the back of the front tires. (see clearances at
https://imgur.com/a/u59eu)
I then cranked the torsion bars to see if it would clear...but no luck. The inch and half or so, of height the torsion bars gain, still is not enough to clear the tires. (see clearances at
https://imgur.com/a/BtWoJ)
The solution is to remove the fiberglass body panel right in front of the doors and trim them. Also, you will need to remove about 3 1/2" from the plastic in the wheel well from the same location. Here is a look at what I trimmed on the body panel.
In the end, I put the torsion bars back to where I started at and everything fits fine. No rubbing whatsoever. I took it out for a drive and am extremely happy with its road handling now. Now more spongy ride and still very quiet.
Before Pictures w/ clearance measurements
https://imgur.com/a/8pba9
After Pictures - w/clearance After, before torsion bar tightening
https://imgur.com/a/u59eu
Clearances after bottoming out torsion bars (3 & 1/2 turns)
https://imgur.com/a/BtWoJ
TL;DR - Largest tire you can get on the GM AWD w/o lift is 265/70/17, with trimming plastic/fiberglass.