Thats probably a fair statement. I can envision a few circumstances where the open chamber would be of benefit, such as lowering the compression to a more pump gas friendly area. I wonder too about valve shrouding in a certain area of lift, as it appeared the 915 trailed the others until a higher valve lift, and then it jumped ahead, maybe the valve finally made it far enough out of the chamber to change the flow characteristics?
FWIW, I have a set of the 516's that many despise, with the 1.74" exhaust added, and a mild bowl port with a 3 angle valve job. They flow in the 240 cfm range at around .500" lift, 28" of water. Thats a fair increase over any of the stock heads flow #'s. The fellow who ported these did tell me they take a good bit more work than the newer port designs, but if you really want the closed chamber....