None. If its HP you're talking about. Your 18's have sapped the horsepower, and there's no gettin git back. That said, changing the ratio from stock (which was probably 3.64 or something) to 4.10 will give you noticeable improvement in acceleration, and also noticeably higher revs at highway speed.
I think installing the diff will acheive what you are after, but you may not be happy with the revs at 60mph. Try it.
18's on an e30??? I hope you have smooth tarmac where you're from.
Post some pics so I can see what it looks like. I've got visions of a rollerskate and would love to be proved wrong. I have 17's and I fiond them too big and am on the hunt for a set of 16's. Something like these maybe:
or these: