Well I am not an expert on the cars really, but, removing the cat makes for a more free flowing exhaust.. So, your aftermarket one, might make a louder noise, it will also increase your BHP slightly (very slightly).. Removing the cat and de-cating some cars make them spit flames.. Depending on mods really.. All in all its just a slight performance upgrade.. Its worth doing.. But I dont know whether or not it affects your MOT..
I heard on Turbo cars removing the cat has no effect on this, as it will still pass the emisions test, but i aint shure...