I think that gas checks would cause an increase in pressure. I expect the percentage of pressure increase to grow with the gas check load when it moves from a reduced load into a full charge load.
A gas check design bullet without check vs a plain base bullet of the same design I expect would be nearly identical, with the plain base bullet having a slight increase in pressure due to larger driving band surface.
Thank you Larry for the testing, and thanks for all those that will contribute bullets, time and knowledge to this. It is a furtherment of the understanding of our craft and hobby.