Simple - you reach the max flow capacity of the either the manifold opening or the adapter and then no gain. I don't know what the flow capacity of the 1.5" manifold opening is, but I believe it's greater than what the stock carb flows.
For example, let's say the 1.5" opening will flow 250cfm, and you top it with the stock carb that flows 156cfm. What is your limitation? The carb. Now, add a 2V adapter port matched to the manifold opening and top it with a carb that flows 270cfm. What is your limitation? The manifold opening/adapter. Was there an increase in flow? Yes, in this hypothetical example, flow increased by 60%, or 94cfm.
There are several people on this forum who can attest to the performance improvement of the 2V on a funnel adapter over the stock 1V, and Doug will attest to how well a much larger 1V (>300cfm IIRC) works on the early manifold. Port matching the adapter to the manifold opening will ensure you get the maximum flow possible with the 2V/adapter combo. Granted, it's not the best option from a performance standpoint, but it's far better than the stock set up.
My argument with Clifford was, and still is, if I'm already limited to a manifold opening of 1.5", why sell a 2V adpater with a 1.3" opening? To me, the adapter should match the manifold opening so the user can get the max benefit possible, not reduce the flow potential. Their adapter for the later 1.75" manifold is port matched, so there's an obvious disparity in their logic with regard to the early adapter.
