I see your point , and from Oettinger its TUV approved too.
However cat back isnt maximising the 'exhaust' component , so in theory the Oettinger kit with Milltek would be even more power! (although without an LSD I'd have to question at what point >300 bhp becomes useable)
That and the fact Milltek has a great reputation , so its bound to be quite an improvement power and delivery wise.
Must be a lot for £1200 though ! maybe its a 4inch bore lol
lets see....