Typically you can get plenty of camber on the rear, especially if there is any lowering at all. I am at -1.9 and I am at OEM USA ride height. In fact when you get some lowering, often the camber is excessive and can't be dialed down without adjustable toe links. Another considering in using GT3 LCA shim type arms in the rear is you don't want to mess up the axle geometry, I think that is why some folks have trouble getting enough front negative camber on turbos and C4s, and there are axle spacers from Porsche that help to resolve this.
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