Your assumption that rolling start locations are specified in each track's AIW file is correct Jempy. According to Bill Pryor's "rFactor AIW Editor Documentation, Tips and FAQs" rolling start positions are defined in the "Teleport" section (rolling starts on my tracks are usually at the "standing" start grid). There is another, somewhat convoluted, option that involves deleting the "Teleport" positions, listing new positions in the "Alt Grid" section and then including the the line "RollingStartLocations=1" in the track's GDB file.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.