Gotcha. I have a Quaife 3.08 in mine, it's a huge upgrade over the stock open diff. Even if it was just for the street, I would do the upgrade again.
If you do the diff, the M3 subframe brushings are a must. The stock subframe brushings are rubbery and allow the subframe to move around a lot .... it is really noticeable under heavy acceleration. The M3 ones also transmit a lot more 'feel' through the chassis. I added the M3 front and rear sways at the same time, which improved the tautness of the chassis. The rear sway is much thicker (read: car fishtails easier, so you have to be careful w/ throttle in turns). Front is same thickness but slightly tweaked. A major difference with both is the much harder end links ... that is what tightens the feel up.
Here's some of my impressions of that setup:
Originally Posted by CJ421
Been driving for a few days now with the parts in, 150 miles or so. Absolutely love what the sways have done for the overall feel of the chassis. It is noticeably tighter, no play. Body roll is lessened. I can toss the car through corners a bit more freely.
The LSD is also great. I got it for the track but even on the street, the difference is noticeable. Power delivery is a lot smoother (probably thanks also due to the M3 subframe brushings) - car pulls straighter under acceleration and seems more planted. Applying some gas through turns - the power just happens, you get accelerated through the turn. Before the car seemed to have some squirreliness doing so.
Going over bumps, ridges, any sort of broken pavement under any sort of acceleration - car just pulls forward, no hesitation or TC lights on.
I've decided to complete the M3 suspension. The front is especially important, it improves steering feedback. Currently have all the rest of the OEM M3 parts sitting in my living room along with the ASTs, waiting for a free week to install.
This is an excellent thread to read on what the M3 suspension components do:
The entire M3 suspension is a fantastic upgrade. New, the parts are $2k or so. And install is about $1-1.5k, depending where you go. That is what I would do if you're OK with stock ride height. They are all plug and play:
You could do pieces at a time. Cheaper labor-wise in the long run to do it all at once.
M3 suspension does not change ride height, so if you want that to be different, you could start with coilovers instead. Personally I would do the M3 suspension first and then go with the coilovers. You have a lot of choices when it comes to coilovers but only one choice for replacing the parts with M3 ones ....
Hope that helps.