I've got a stage '2' (270bhp ish) standard MK5 that had a decat downpipe when i bought it. I had a 200cel magnaflow cat welded in and can't say i noticed any difference in performance.
I had mixed opinions on whether the car needed the map tweaking to suit the addition of the cat, the company that did the map insisted it was required (of course they would).
All in all it was a bit of a hassle getting the cat fitted etc. and also the extra cost.
I'd try and find a new mot tester who likes £20 notes.