I really think GTI owners should get off the "get a rear bar first thing" band-wagon. The issue is with the front of the car, not the rear.
Agree with David. I know we're talking about keeping things street-able, but I want to add to this.
It's a really common thing for fwd people to apply the same thinking and setup as a Miata (50/50 weight, perfect suspension). The rules and trends only work when you are starting from an equilibrium point (does not mean equal roll stiffness). Unfortunately most suspension tuning material is based on cars that are well balanced and rwd.
All cars are setup from the factory to understeer, especially fwd. FWD cars always have a higher rear roll center, so the roll axis is nose down. Guess what, the front tires are easy to overload. If you are already overloading the front, more rear bar makes it happen sooner. The front must be propped up to have a good handling fwd car, especially when it's heavy.
Some perspective:
Common STX setup is for a twin is 400F/400R, 22mm front bar/softer than OE rear bar, strut front/multilink rear. Yes motion ratios are different, but let's assume front roll stiffness is similar enough for comparison.
Corner weight for each front on my BRZ is nominally 725lbf, total weight of ~2600lbf
What would you expect the result of adding 300 pounds to
each front corner and not changing the roll rates would be? Reasonable to assume more understeer? Reasonable to assume you'd need more front roll stiffness to keep the balance?