google rear sub frame failure and you will see what happens with the power you making it will definitely happen to you

basically you need to reinforce where the sub frame connects to the body by welding in plates... its not hard to do but you need to strip the back of the car to do it so you have to drop the sub frame anyway...
while the sub frame is out you can mod it to fit the M3 diff it is quite a big job:
your subframe:
M3 subframe:
as far as i know the sub frames are a direct swap, im sure a quick search will tell you if it is
you will also need to modify the flanges on the diff to fit your shafts...
this is a lot of work so i have decided to get an OS giken lsd (clutch type)... but depending on what you want to use the car for a Quaife (gear type) would be a better choice both go for about 15k...
you will need a diff with the ratio you want to run to fit the lsd inside (3k)
those are the options

the thing is you need to decide what you going to do so you only drop the sub frame once its not the kind of thing you want to do often
I have decided to go with a OS giken lsd and save myself the trouble of modding the sub frame