I posted about the diff, and it's not quite a huge difference, but it is significant(3:15 to 3:23).
I feel this is the best mod you can do to NA car.
Next car I buy, screw the chip, near gears will wake the car big time!
If you drop a 3:38 into your M3, you will access the power so much faster, it will give the illusion that you have 40 or so more HP than you actually have.
In my 325is, I dropped in a crazy 3:91 ratio, and I was beating M3 in acceleration. I must have had maybe 220HP to the crank with all the other mods... Of course you suffer on the top end, and cruising speeds, but who spends that much time doing 130+ on the highway anyway...
The new M3 is so well geared with the 3:64 plus the six speed, that there is no way you can hang with them unless you have forced induction, or serious internals done...
The technology inside the E46 M3 diff is tremendous, especially for getting optimal power to the ground when launching...
It's a big car, but it IS brilliant!!!
