Its worth noting its hard to judge what a Back box or system will sound like without viewing one on the same cc car as yours.
Also its always hard to judge how much difference losing the centre box will make to the system as yes it WILL increase noise levels but alot depends on the back box you are currently sporting.
Decats will def increase noise another bar as the cat acts like another silencer in the system so if your thinking about a single box layout (just back box) with a decat I'd really expect to get some attention and the danager with this is if you attact the attention of the Old Plod and they look closely or give you a provider to vist a MoT station to have the car checked you will either have to refit your cat or fail the test and get told off
Imo decats are best of track cars as they basically drive to the track, run, then come home.
As for whether manifold changes add any increase or change is open to debate to be honest.
