On the cheap i'd stay with the stock k03s, fmic, silicone tip and a remap for a decent 220-230bhp. K04 means buying more bits changing oil lines and getting s3 gear getting you probs 260-270ish without a cat, If getting s3 gear Id get your k03s hybridised for £600, then get correct running gear then a remap, but sadly its stage 3 so like £600 but thatd give you about 280bhp
People on audi forum used to get the same figures as people who spent on exhausts, but probs felt less free reving, but with k04 and k03s Hybrid youd deffo need a bigger exhaust.
Depends what size you got now though?
Anything more than the k03s I think would need a an lsd. Depends how much moula ya got.