The rear or the car is a straight axle. Without some seriously heavy equipment and a butt-ton of $ there is no way you are going to adjust the camber on the axle. Same goes for caster, and, in fact, everything in the rear.
Correct, there are no listed rear alignment specs, at least not for North American cars. Not worth it in my opinion, at least not for ITB. I tried it for circle track, didn't notice any better grip, only more even tire temps. It wore axle bearings like a MF though...
I would have guessed that the reliability issue would out weigh any performance advantage. On the 1st gen rx7's however seeing as it so hard to keep them from oversteering it may be an advantage. I have a friend in Australia the campaigns a Volvo 740 turbo in a touring car type series. For this season he decambered his rear wheels two degrees and was faster on all track he runs. He had no reliability issues all season. His website is www.ovlov.net . I do not find that handling is a weak point for my Manta as far as my competition goes.
How do you "de-camber" a live rear axle? The RX-7 has an IRS, so I can see how you might adjust that. But the only adjustment available with a live rear axle is "stagger" (which wheel is ahead, or kind of a rear-wheel steer "pre-load"), which is just a circle track thing isn't it?
While the solid rear axle of the Manta has no adjustability for camber. I have heard of people using a torch to spot heat the axle tubes to permanently change camber. Once upon a time, I was told that 1/2 degree of negative camber could be added with no ill effects. Don't know how this compares with RallyBob's alignment changes tho.
The first generation rx7's had a solid rear axle. The two methods of bending the tubes on a solid axle car are to spot heat the tube on the top and it will shrink slightly as it cools creating negative camber. that method is rather hard to control. The other method that I have seen circle track shops use is a press. Far more reliable and accurate. With you car sitting at static ride position on a level floor you tack weld a small plate of metal dead level on the top of the axle somewhere. The axle assembly is them removed from the car and placed in a jig. The axle assembly is then rotated with a level on the tack welded plate until it is dead level. the axle can then be locked in that position and bent with a press to reach the desired camber without introducing and toe change.