Powerflex has inserts that fit inside the cavities of the existing bushes, effectively stiffening them up without the major labour cost of pressing out the old ones and in with the M3 ones.
Marco-Polo at babybmw.net discovered these: topic on babybmw.net
Here are the parts:
Rear subframe front mount insert
Rear subframe rear mount insert
I don't know where you can buy them, nor do I have any experience with them. Just thought it's to good to not mention it if you didn't do the M3 ones due to cost, or wanted a more reversible solution.
Oh, I'm guessing that in the long run there is a small risk for wear due to internal friction of the insert/bush if used hard, but should be OK for street driving...