Dont know about the performance gains of a full system, i imagine it will give you a little more top end ooomph but thats about it, but get a stainless and it will out run the life of the normal pipe 10fold. Ive only got a backbox on the standard mid pipe with a decat, it did free up some power & noise.
The decat is a worthwhile addition, remember the MOT thing though but its not hard to change, may be a b*****d to get off the cat the first time if its sezed on there ( the four bolts i mean) but any garage would cut them off and replace. If you want a loud (er) exhaust get a unsilenced if you want to remain a little quieter get a silenced. I can reccomend Ktec for either of these...