AFAIK, limited slip differentials only have problems with differing outside diameters, so as long as the tire was the same outside diameter, you'd be fine (the Porsche inflate-a-spare is one way to accomplish this). Moreover, I had a space saver spare on my Mustang that was not the same outside diameter as the road wheel, and the Mustang's LSD had no problem with it, at least for the short time that a space saver is designed to be used for. I think the LSD argument is a red herring.
The offset argument is a bit more compelling, but again, we're not talking about a spare that has to be capable of handling the Nurburgring. It only has to be capable of getting you out of a tight spot. As long as the tire physically fit and didn't catastrophically affect the handling to the point where the car was undriveable, a compromise in offset would be fine. WRT to the M3, I bet a Type 44 8 inch wheel would work just fine for both front and back (for temporary use); the M68 8.5 inch wheel works on the back (and is used in Europe for snow tires). The M5 uses the same wheels, IIRC, as the 540, so the 540s spare should bolt right up.
The Z3 may honestly not have room for a dual exhaust and a spare, although I tend to think that a pair of three inch pipes running next to each other in a true-dual arrangement would a) run in the same or similar physical space as the Z3 3.0 arrangement, and b) could be tuned to have nearly zero backpressure. But even if it can't, how to explain the M5 and the M3? Both of these platforms have plenty of space for a phalanx of straight pipes in addition to the spare tire well.
Pinecone said:
You probably haven't ever used Fix A Flat. 
Let's see in 30 + years I have needed a spare twice. And in only one case would not having had the spare been a problem. And in that case the Mobility kit would have been fine.
WRT the exhaast system, it isn't the four tips that make it so large, it is the full dual exhaust system (MZ3s have no room for sapre due to second muffler). Also the manufacturer has to build to a different standard than the aftermarket. Noise, durability, etc. Look at the Eisenmann for the M3, no lighter, no smaller than stock, but meets German standards.
As for the tire being toast with using the Mobility kit, I don't use plugged/patched tires on high performance vehicles. On our Jeep, maybe, on one of the BMWs, never. It isn't worth the potential problems for the few hundred dollars saved.
And you didn't address the problems of the different rim offsets front and rear or the problem of a smaller spare and LSD.
Some of the MZ3 Coupe guys have found a Porsche shrunken spare (you have to inflate first) that fits with a spacer. No idea how the LSD likes it. For an M3 you could do the same.
Personally I have no trouble with no spare.
