right, physics is simple:
if you increase the rolling circumference, for the same axle rpms you'll be driving faster, BUT, this also means that due to bigger radii you loose the torque. Vice verse with the smaller circumference.
again, as long as you stay within +-3% should be ok.
guess that TÜV gave me first time hard time ONLY to force me to do special check and get "Sondergenemigung" (well, special license) for them and release my pockets for some 94 EUR. Calculations were OK, BUT I guess that brings them more money.
regarding the clearances to the arches and suspension, this Pirellis that I have mounted look like having no problems. I have 3.2JTS Q4 SW Ti which is factory lowered, so it looks OK for the moment. need to check it though once time comes for change, since then I will definitely putting some more sporty suspension.