Contact the Tech department at Bub Enterprises; the folks that make the Rinehart True Duals. They have probably worked with virtually every engine modification combination possible with the HD twin cam, and can tell you if Air is a problem.
You might be on the right track. The HD dealership I use recommends not installing Rinehart True Duals on heavily modified engines.
Also, I am not familiar with the throttle body you selected (I haven't cracked the case on my 04 EG and probably won't for a year or two), but when doing the Rinehart True Duals, Stage I a/c, and Terry Components Terminal Velocity II installation, I spoke at length with the engineer at Terry Components and he was very specific about the stock throttle body being a big limiting factor on horsepower and torque because it cannot handle enough air for seriously increased cc's, cam lift and duration, etc.