In the end it probably doesn't matter that much, like I said the valve is only open about 20 seconds per minute of operation.

The losses to a dual plane are due more to the high RPM wall friction (turns in the runners) compared a straighter shot single plane, even if the CFM and cross section is the same.

That said the 2D is an awesome manifold, the only reason I would NOT use it is if the upwer bottom and mid range torque was excessive for the body weight or the gearing/stall/shift recovery RPM of the combo was higher than the torque peak achievable with the manifold, not gonna be the case in most street driven 500" motors in a B or E body, up the cubes to 540-572 and/or lower the weight a few to several 100 pounds and the MP '337 is likely a (slightly overall) better choice.