I have over 10 years experience working with an MDF vacuum table but I have no data points. I do know it flows better when it is skim cut. I know this because I have a commercial router with a gauge in the chamber and when the 1" waste board is not cut, the gauge will read a higher vacuum pressure than when both sides of the board are skimmed. Higher pressure in the chamber when the table is wide open indicates a restricted surface. In theory, a guage below an open table with no restrictions will read next to nothing for vacuum pressure. To me, this is the most desirable condition. Effective distribution is also important.
The best way I have found to seal unused surfaces is with clear plastic packing tape. It's cheap and very effective.