So, if my car is stage 1 remapped and I put a non-OEM filter on (in line with that suggested by the OP) would I see another (approx) 15bhp+ or would I need to do other modifications?
....Firstly just to clarify, I'm assuming you mean a non-OEM high flow tubular intake with filter and not a flat panel filter which drops in the OEM air box. I say this because a flat panel will not deliver substantial bhp gains regardless of remap. I have run an ITG flat panel filter on Revo Stage 1.
Regarding your question of 15 bhp+ or not, you don't say whether you are running a K03 or K04 turbo'd car - The K04 will probably deliver more.
A high flow performance exhaust such as a Milltek, BCS, APR, will make the difference to release the power too. Also, Stage 2 gives more than Stage 1. All I can confidently say from direct experience is that the combination of a remap Stage 2, performance exhaust, and Twintake, on a k03 car should deliver 15-20 bhp extra.