Forget about the wheel bearings, it's the handling I'd be really worried about, it might look smart, but it'll understeer like a pig as there will be a lot more grip at the rear than the front. The only front-to-rear wheel difference I've ever thought was worthwhile was to use same spec wheels, but put the rears on very slightly NARROWER tyres than the front, so give yourself slightly less understeer.... and then put the rear wheels on spacers to give yourself that wide hot-rod stance at the back: there's a few FWD GTI's that have done it (Polo GTI and the top spec fwd A1). It'll increase your rear track without increasing your rear grip. Only difference that you'd have if it was a S4 would be that you'd trash the torsen differential... quattro's can't handle different spec wheels as the rotation is different on each axle and the differential will be working overtime. RS4 body on a fwd drivetrain might be interesting and would make this a doddle, but that's a whole front-end conversion too as the wings would need to match the other body panels...
