I got the SALK fitted recently with the rear GTI ARB as well. My factory fitted RARB was 1 mm less thick than the one from the GTI.
I think you'll love the ALK, I recommend the SALK over the WALK because the SALK seems to be tougher in the reviews. You'll improve your understeer a lot, really a lot. I'm loving mine.
The RARB will allow your rear to slide more in the tight corners, decreasing the understeer as well. I preferred a rear ARB from the GTI because I wanted to keep rubber bushes. Instead of getting already a stronger RARB you can try fitting Powerflex bushes. That might be enough for your needs and it's much cheaper. Purple for road use or black for track use.