Best thing you could do is what i have done with mine install a heavy duty rubber band turbo and some golfgti.co.uk number plate frames this should give you an extra few horses and dont forget to add a few stickers as well they are worth at least 5 horses each.
I would stick to the optimax fuel i doubt that adding fuel additives will do much for your engine long term i know ive tried makes it go a bit faster but the stuff i used was ?15 a bottle so not very cost effective.Just run a bottle through every 2k and chuck out all the seats you dont use cos if its a 5 door like mine its prob 0-60 in about 3 years.
You could get rid and get a 3dr mk2 16v still the best golf out there but the good ones are gonna go up in price soon i recon...
