I use those sizes on my Mk6 gti - 28mm front and 24mm rear. Basically the same car as a mk5.
Handling is neutral for me; set to soft on front and rear. I three wheel going up steep driveways etc. with all the stiff suspension work/chassis bracing. I've never competitively AutoXed, just for fun every few weeks in the summers, so it's hard to say if this set up is better than stock but it is absolutely better than the APR-branded Hotchkiss bars I had previously.
I love the bars because of the hardware and greaseless design, unlike the Hotchkis bars. I sort of wish I had a smaller front bar but I'm not rebuying parts I've already spent a lot of money on.
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