You would be better with 550's but to be honest in my opinion you should be looking at a mappable ECU as a matter of priority.
I am assuming you are using the standard ECU, but have changed the downpipe, induction system etc etc but still using the standard ECU and map not the best idea.
You should be able to pick up a TY754VN4AA box out of a blobeye WRX for around £300, poss another £100 for the diff at worst if you cant get it thrown in with the box.
Clutch, might be worth looking at the ACT range from the states, they are getting good revues and work out fairly cheap. Other than that that its an uprated exceedy.