The sdt set up is crap, even on a std engine, has nmore to do with emissions than power.
You could try packing out the seal first by cutting a cereal box up into a ring shape and fit betweent he cap and seal so it exerts more pressure.
I would just get a decent breather though, don't need anything fancy, look on RATSPORT.co.uk around £50.
If you want a custom made one let me know, and it can be made and hidden out of the way.
tabetha