Are you going to be going for stage 2+ come time?
Honestly, there is a little more to making a car fast than a decat pipe vs a race cat pipe.
Are you planning on doing all of the following as well as the downpipe?;
Uprated cat Back part of exhaust
Intercooler
Fuel Pump
Intake
Lighter wheels
Uprated suspension (coilies bushes etc etc)
Diff
Uprated Clutch
Lightened fly wheel
Strip interior
Remove A/C
Lighter 2 piece brakes
etc
etc
etc
Any one of them will make more difference than a decat over a race cat I reckon with the exception of the cat back part of the exhaust and the clutch.
Having seen a stage 2+ Cupra against a stage 1 Cupra last night, there is not a night and day difference between them, and that is a difference of 60bhp. On the road, you wont see any difference between 200 cell downpipe and decat in my humble opinion, but a car inspector would, as would your insurance company in the event of an accident