Without really knowing the answer ... I'd say that's a track by track modification that should be needed.
The grid places on track are mentioned in the .AIW file .... so that's surely there that it may be modified ... but does it need a track 3D modification ? .... I really don't know.
Someone here building tracks should know the precise answer.... I doubt it might be a hard-coded feature as a bunch of tracks were modified to allow much more cars on the grid.
Anyway if I start from the base idea that the rolling start distances are a result of the distances between the grid places.