Both of these pics, AFAIK, are classic examples of throttle application -> rear end squat -> increasingly negative rear cambers -> rear tires ride on inside edge of tread.
I sort of noticed it happening on my first set of tires, got 30k miles out of them anyway. I watched it happen on my second set of tires rotating every 6k to 9k miles. Once the inside edge gets flat like pictured in post #1 the flat area grows faster and faster, you can get from #1 to #8 in maybe 10k miles or so.
Rear camber eccentric bushings will help. A rear cradle alignment may or may not help, depending on what your static rear cambers are on an alignment rack today.
It is your dynamic rear cambers that lead to this tread wear, working from your static on the rack cambers is kind of a rule of thumb guide that depends on how 'average' you are about stabbing the throttle.
The other fix is to increase your rear spring rate, so the rear end squats less under throttle. Lots of GTO owners are doing this with drag bags, I am not one of them. The other option is to go with stiffer rear springs.
On the plus side drag bags are cheap (about $100) and it seems like drag bag owners are inflating the drag bag just enough to dial out wheel hop. Doing so
can dial out some existing suspension bias (understeer) to help the car enter corners better.
The down side is A) drag bags will eventually leak, usually starting at the worst possible moment and B) while I have not ridden in one personally the same alteration in suspension bias that helps corner entry is
likely to detract from corner exit.
I did ride in a GTO with stiffer springs on the rear axle only and did observe much the same double edged sword described above. The car got squirrelly coming out of corners.
If eccentric rear camber bushings are not enough by themselves, the better option is to replace all four springs with something stiffer.
The downside is replacing all four springs is considerably more than $100 in parts, and the labor to change the front springs is not insignificant.
The up sides are twofold: 1) instead of suddenly starting to leak deep in a corner when your knuckles are already white, springs will gently overtime. Second, keeping the front and rear suspension in balance to each other will allow the car to enter, apex and exit corners very predictably.
So there you have it. I can't think of a single user here with one negative thing to say about drag bags, but I can't think of one who has a leak yet. Actually, there was one guy who had a squeaky noise, but he got it resolved.
Several vendors on the board sell springs, frequently packaged with the major bushings that you might as well replace while you have the suspension apart. Whatever nits might be picked between the dark blue bushings and the red bushings and the light blue bushings, opinion here is that all are vastly superior to stock GM parts.
For further reading, Mike White figured this out about a year ago:
http://www.ls1gto.com/forums/showthread.php?t=50120&highlight=rear+camber
HTH,
S