In terms of handling, all the important parameters like roll center, scrub radius, etc. are all referenced to the centerline of the wheel/tire, so offset is not affected by wheel width. That said, if you want wheels wider than 8.5, ET34 most likely will have trouble clearing the struts. Some folks are willing to go with less offset, in order to mount 9" or even wider wheels, and just live with the consequences. IMO, not ideal from a suspension geometry POV, but different folks have different priorities.
To me, wider than 8.5 up front is rather pointless. For a square setup, 8.5 will easily accommodate 255, so rear wheel power handling is plenty for
3.3T stock HP/TQ or even a bit over. If you've modded up your engine output quite a bit more, then you might need wider tires out back to handle the added torque. For 2.5T and 2.0T, wider than 255 isn't necessary.
For street, our
G70 6MT run 255/40R18 square on 18x8.5. I prefer offset around 38-40mm, which are perfect for the rear axle, and just run 3mm or 5mm hubcentric spacers up front to correct offset to within 1mm of stock 34mm. You could also just get ET35 all around; that'll work just fine.
