DJM above would be best local option

GForce in the states already offer some ratios

http://www.gforcetransmissions.com/tran_gt-5.asp

http://www.gforcetransmissions.com/t-5_ratio.htm

Samsonas do some "universal" rwd dog and sequential gearboxes with a variety of ratios

http://samsonas.com/?page_id=234

Universal seems to mean a BMW fitment with an adaptor.

I think SQS used to do some T5 stuff, but havent seen anything listed for some time

http://www.sqsracing.com/aktuality


A diff swap not get you near where you need to be ? Certainly be a lot easier and cheaper ?
Or try and hunt down a Tractive gearbox ?