It's hard to know, on these lowered cars, whether when it "bottoms out" you're hitting the suspension stops or bottoming the shock absorbers. If you have "stock" shocks, they're expecting more suspension travel, simply because the original VW design sat 3" or so higher than our cars, so the entire shock absorber is longer. I took a quick look on several parts sites and did not find any rear shocks for "lowered rear suspension" so I can't offer much advice on that. Everyone sells shorter shocks for lowered front ends but not rear. That would make it more likely that you might bottom the shocks by full compression before hitting the suspension bumpers.
Something to think about.