I just swapped out drives on a 280 SS. from BIII to BI. I think the 40 also has BIII's on it... you could probably buy the boat, take the drives off and sell them and buy a nice used set of Bravo X or XR's and be even or ahead money wise... the BIII's seem to command a big price used where the Bravo 1 series are more abundant and are cheaper used.
A member on another site said he picked up 5 MPH going from 3's to 1's on his boat. I haven't tested the 280 yet but if we pick up anything it would be pretty sweet.