Well you can go dinan they have some good software but also look at BMW themselves they make a nice power kit for the N55 engine that really gives it some pick up.
Not exactly sure what you mean - there are numerous aftermarket products available of all kinds: suspension (to include shocks, springs, coil-over kits, bushings, anti-roll bars, etc) , brakes (large brake kits with rotors & calipers, or either, stainless brake lines, etc.), wheels in numerous varying combinations of styles, diameters, widths, weight, etc., interior, exterior, engine, you name it. The answer to your question is, it depends on what your performance goals are, &/or what you want to do, & how much money you want to throw at the car to get it to meet your goals.
BMW performance catalog if warranty coverage is important to you. Dinan would be the only other route that may be warranty safe or offering a matching warranty. Otherwise there is a huge amount of aftermarket support for whatever your performance goals are.
