From what I have researched before getting mine... in an MKV GTI magazine shootout between several tuner GTI's from several years back... for the most part they ran the smaller 26mm bar up front set to soft & the larger 24mm rear bar set to hard.
H&R runs all their GTI's with the smaller 26mm bar up front set to soft and the larger 24mm rear bar set to hard.
White James recommends the smaller 26mm bar up front set to soft and the smaller rear 22mm bar set to hard.
AWE's MKV time attack GTI only ran a special 28mm rear H&R rear sway bar... and that thing is fast.
I'm sure APR runs their in house bars... but I don't know what they set theirs at.
Me... I run just the bigger H&R 24mm rear sway bar set to soft with OEM up front... and it works just right for me... in the future I'll upgrade the front and run soft/ hard like the other setups... but for now I'm really happy with this setup... handling is nice and neutral, no snap oversteer, just a hint of controlled oversteer through fast corners which is just the way I like it... and it reduced overall body roll
Figure the harder the rear bar is = more oversteer... the harder the front bar is = more understeer