from personal experience, in my opinion avoid powerflow and the same likes! The thing with these places is there cheap £300/350 for a cat back exhaust, pretty much any tailpipe design you like. Although they are only cosmetic. I had a mine done (stilo) and had RR a few months lata and nada gains. I suppose it depends if your after HP gains? It sounded nice and let the engine breath more freely. There not professional turners, the welds are done on a get it "in and out" as quick as possible bases and the profiles of the angle/bends arent calulated. There just welders basically, not that theres anything wrong with what they offer and grades of steel used will out live the car.
Go for something designed for the car and model, ie. the likes of supersprint which there products are proven by RR! Although i know SS isn't avaliable yet for the mjet!
there are alot of variables that can effect the gains achieve, pipe diameter, etc... the large the diameter isn't actually better which most "custom" places try to tell you. Had along chat ages ago with pete (oldschool) about this when i had my stilo. Personally any custom made ain't gona do anything apart from the cosmetic and the GP's STD exhaust ain't that bad its just a waste of mulla untill the like of SS come up with something.
There are some good custom build places out there, but the cost is alot more like over £1k+ and they'll need the car alot longer, a week or more for testing not just a few hours on some friday afternoon.
Again depends what your after and your budget...
