If your gonna keep the s1 for the rest of your life and never sell it then consider doing the mtx conversion as it will be cutting or grinding back the inner wing of the car. if you ever want to put it back to standard then there will be a bent inner wing or cutout inner wing. the way s1 are going they are becoming alot more expensive and a much rarer car then s2 or fiesta rs turbo's.
or you could get a late big bearing box and have the ib5 gears and bearings fitted which can take upto 250bhp - there a company called GEARS that does such a box.
i have the same recently fitted and it is much better than the standard box.