To be honest, if you have just a K&N and cat back a remap is not really worth it if you are planing to do even more mods later on. I would save up and get the manifold, sport cat/decat, etc, and then get a remap. In the long run you would have saved that £350 that would have bought the manifold...
I had a remap after just fitting a cat back and to be honest the power increase was not noticeable enough from standard for me to really go WOW! Whereas if you're jumping from 150 to say 180 with cams, manifold, etc and a remap then you will notice it and it's much more worthwhile.
Obviously though, if the induction kit and cat back is all you are planning and you want to sort out any fueling issues, then definitely go for a remap!!