I have tested the HPI 55, 58 elip, S&S 66 and the HPI 62 on on my 124r with 662-2 cam. In that order. I saw no more power with any of them. TB wasn't the choking point and I have a streetable redline. 6200. Parking lot drivability, TB noise, and throttle response was a big difference in the test. The 66 was the turd. More to TB's than the size of the blade. Shape of the manifold, bell mouth, and bifurcate will all play along. The HPI units being a 2 piece is also an advantage.
Sounds to me like the dealer put you into one of their most expensive part builds that in the end will be a waste of money that wont put out any more than a well thought out, bang for the buck, proven combo build.