I have the Pipercross filter and Cobra backbox fitted to my NC, and I’d be lying if I said I felt any noticeable difference.
I’m sure there’s a perceived improvement which is down to the extra noise (which does sound pretty good IMO), but even with a remap I’d suggest you’d struggle to see more than a 5-8bhp improvement, which in reality would probably be at the upper end of the rev range.
This is personal opinion based on other N/A remaps I’ve seen though, so I could be way off the mark.
What Keith says, you really do need to replace the manifold with the cat in to see the best results. It’s very restrictive as it is, I replaced mine with a Racing Beat one from Paul (Roddisons) along with the air filter and Cobra back box and remap from Clive.
The car is a totally different drive now with a very healthy increase in power.