On N/A cars a decat isn't always beneficial purely because N/A requires some exhaust gas resistance to aid the scavenging which helps clear all exhaust gases out of the cylinders.
However it is the opposite on Turbocharged cars. Downstream of the turbo you want as little restriction as possible (decat/big bore etc), so the bigger the better. Though you do get to a point of diminishing returns in that you will find that, for E.G a 3inch system causes little to no resistance so going bigger would be of no benefit.
So though decat is better for outright performance the better option (IMO) would be something like a 100/200cell sports cat which, as said earlier, gives performance benefits with the assurance that no faffing around changing exhausts is need come MOT time. Like Smurf said propshaft drop is needed and it's not the nicest of jobs (even with a ramp and all the tools)