I think part of the reason for USGT to be non boosted 21.5 is to give a natural progression up the power level.
VTA 25.5 non boost Vintage bodies
USGT 21.5 non boost Modern GT bodies
Stock TC 17.5 non boost TC bodies
Expert Stock TC 17.5 boost TC bodies
Makes sense to me and gives structured classes that anyone can find a place to race in. Also gives consistency between tracks.