When did you play GT7?You're talking about photomode car polycounts. You don't need even 500k poly cars for GT7/PS4 gameplay, more like 200-300k. Driveclub's cars are 250k during actual gameplay.
GT's premium cars are not behind for actual GT7 gameplay, they're ahead.
GT6 has ~445 premium cars, those should be ready to go in GT7 at next gen quality. Any other cars they can get up to that level is great.
If you actually think Forza 4 or 5's cars are a million polygons during real gameplay...