I'm having my transmission rebuilt and the splines on the CV shafts are worn. I'm seeing aftermarket replacements priced from $50 to $150 while original MINI parts are over $600. Is there that big a difference in quality? Or any at all? This car is primarily a track car so I don't want to skimp on quality.

 

Cheap aftermarket CVs / axles are crap. Don't waste your time or money.
There are lots of aftermarket "race" axles that are good quality and much cheaper than original MINI.