Hi Wilf (sEG) , 
Let me venture a guess here: your air filter element isn't the bottleneck
An engine is in some respects merely an air pump. If you want more HP you need more airflow - of course it helps if the air contains a bit of vaporized gasoline (or is that vapourised petrol
).
Since the removal of the filter didn't change any power readings, my guess would be you have some other limiting factor. Compression, cam, ports in the heads, manifolds etc etc. In other words, if you want more HP maybe a supercharger?
As to space, if you look at my gallery some would see "a scoop big enough to drive a VW into"
where others would see HP
Maybe not as elegant as I'd like but certainly functional
Tom