Im not so sure these mods make much of a difference TBH, ok maybe a little, but nothing thats gonna make a massive improvement.
I only swapped my intake pipe for a Ramair one because i got if off ebay for a steal and i think they look a bit better, a bit of underbonnet bling!!
I wouldnt be hacking my air box about either, im very much whatever i do to my car i can revert it back to standard without any evidence.
Youd be better off spending the equivalent ££ in super unleaded and enjoying your car as it is.