Answers:
1) There's no 'optimal' width, but there are correct widths based on the width of tire, as well as what will fit on the car (combined with wheel offset). That said, 7.0-8.5in widths with appropriate offsets (mid 40mm) and appropriately sized tires will work best.
2) Staggered setups (but with the same overall diameter F/R) are good for 2 things: cosmetics and handling characteristics. You can't run taller tires in rear like a drag car for various electronic and mechanical reasons, so you can't get a 'raked' look from the side, but wider rears typically look more agressive from the back. As for handling, wider rear tires give more mechanical grip, thereby shifting balance towards understeer. How this works exactly with the xi AWD setup, I'm not sure.
3) Changing tire widths on a given width of wheel depends on how much of a width change the given wheel can accomodate, as well as finding an overall tire diameter that stays pretty close to stock.
TireRack and other sites cover this stuff in lots of detail, but here's an overview of just the 17in options:
330xi stock: 205/50/17 on 17x7.0
330iSP stock: 225/45/17 on 17x7.5 F, 245/40/17 on 17x8.5R
325iSP stock: 225/45/17 on 17x8
Interestingly, I'm seeing that for the middle setup, 255/40/17 is actually a closer diameter match to 225/45/17, which might be more crucial with the AWD. 235/45/17 and 265/40/17 may also work, but without proper offset the 265s may rub.