I don't think it's been clearly demonstrated that Nationals being an option for IT cars would decrease Regional grid sizes. It might well be that the attraction of the occasional Nat'l race (particularly in weaker regions like the NW) would be enough to sway someone to an SCCA IT car rather than some of the other options with other sanctioning bodies. Or the option of "moving up" might make IT more attractive for some drivers who currently commit to Production classes.

K