Mini GT

I haven't bought any, but from what I've seen they're mostly collectibles.

Having seen how MiniGT cars are assembled, getting the axles stable is like uphill ice skating. While HW/MBX have clips and a way to secure them through various adhesives, MiniGT has nothing close to this in the base alone to assist in a modified build. This brand was never manufactured with an eye to track performance and it does show inside. Bases for MiniGT are totally flat on the interior, with minimal clips at the outer edges to hold axles of lengths not always of the length we expect in racing, so you would have to custom tube the axles for a decent fit

I also want to turn your attention to the NASCAR event at Tiny Track Cars. In the event post, Nick gave a heads up that while Lionel Diecast brand cars look the part on the outside, and may have aesthetically been perfect for the race theme, their inner construction held more challenges in building compared to Hot Wheels

The point is, getting top performance from basic diecast is itself miraculous in a way even from cars purpose built for track play. You will gain only headaches from modifying display quality adult collectible models