1) Less understeer, wheel/tire rotation ability.
2) More understeer, less tramlining, better acceleration/braking (less mass), better gas mileage (less mass).

edit: The OEM style 87 rear wheels when used on the front, (being 10.5 +30ET) come amazingly close to the front part of the fenderliners at full steering lock (outsides of the tires only... the insides of the tires at full lock have room to the rear part of the fenderliner), if you happen to consider running 20x11 all around, consider the offset for the front, and space out the matching offset rears accordingly.

I'm not sure what your intentions are, or if 20x11 all around could fit and still get around the steering and flushness/appearance issues. 10.5 +30ET in front will require fender flares, and there is "some" (a little bit of) room before hitting the suspension.

O/T:

Just thinking out loud now.... to fit 20x11 all around on the E53, the offset would have to be perfect... it *might* be doable, but definitely with flares. A width of 11" with +34 to +36ET would be *close*... higher than +36 sits the outer edge further inboard than the outer edge of a +30 10.5", so going higher than +36 wouldn't be neccessary as the +30 10.5" looks great with flares. BUT... the lower you go in offset, more of the front part of the fenderliner gets eaten by the outside of the tires at full lock. 11" +36 up front would look SICK, but that might hit the suspension and/or rub on the flexible part of the brake lines. At the end of the day... a 315 tire is still going on it (and should be accounted for along with the offset), so... it *should* be doable, but all that work is being done to fit something with more mass and the same contact patch when there is already a solution for using the 315 in the front. Sorry for rambling on...

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Previously owned: '03 4.6iS Dinan Supercharged