depends what reg your car is mate whether you can run a decat permanently or not, think its something like L reg and previous thats ok, (i will stand corrected on that though)
otherwise if you get pulled just say that the car has got an mot and you bought it like it and didn't know the pipe was in there, then you have chance to just get it checked at a later date by which point you would have the cat back in...lol
as for performance, i think it makes quite a difference on the valvers, mine revs more freely and is so much better lower in the range, it felt rubbish when i had the cat back on for the mot