Just a data point for the discussion.
Following the recent change to the BRCA National classes, the Shootout Series offers the same classes for LMP12 cars with 13.5 blinky as the Stock class and 6.5 blinky as the revised Modified category. The switch to 6.5 blinky in place of Open Modified appears to have bolstered entries for all events, with nearly 3-times more competitors signing up to race this class than when it was run as open motor and speedo previously.