Air leaks...........pffffffffffft welcome to the world of MFI on a CVH mate

possible air leaks:

1) THE WHOLE ENGINE ITSELF
2) boost pipes
3) intercooler
4) brake servo

etc etc etc

As a first step i would remove the breather pipe where it enters the metering head/air box assembly and block the hole, and see if any significant improvements are seen (more than likely you will)

If so you will have an air leak somewhere on the engine which isn't an uncommon problem - dip stick, rocker gasket, sump gasket, oil cap, crank seals etc.........i wish you luck