Thought that I would share my experience with these MagiGlide castor cups. I put three of them under a 6'2"/188cm grand piano, weighing 616lb/315kg, and it moves more easily in the MagiGlide cups than on the castors alone! The black cups go well with the black piano, and my only prior concern about the grey colour was misplaced as the grey PTFE isn't visible once the castors are in the cups. I would definitely recommend these to anyone looking to use them with a similarly (or smaller!) sized grand piano! Only difficult part was lifting the piano to get the cups under the castors :-)
My one caveat to this review is that I do have a hardwood floor so I don't know how effective they would be on carpet.
I hope this helps other piano owners who are looking at this product.