[MJTSUB]

I had a e46 330i with the performance package and I swapped the diff in for a 3.46 from a 3.07. It made a nice imporvement and I would have a went lower (numericaly higher) had I had the chance. I think a 3.73 or 4.10 would be an excellent improvement in acceleration for your car. Expect 1st gear to be over very quickly and you will have a hard time getting the rear tires to hook up withou LSD with either gear ratio. Also 60 mph wont be reached in 2nd gear making your 0-60 time longer since you will have to shift to 3rd but who cares. With either gear ratio I don't think you will have that rush of speed that you get from the S54 motor. The power to weight ratio will still be favored on the M3 and the motor is simply more powerful. I would recommend forced induction to take care of your needs.

 

[MJTSUB]

No problem. Same as a manual box.

Car would go thru the gears normally but top speed in each gear would be reduced.

No the automatic is a different length than a manual transmission therefor you will need a custom driveshaft made to fit a M5 diff as there was never an automatic offered for the M5 hence there is no OEM part.