I went for Delrin for my RP6 after a call with the owner of Fidelity Design. The main reason I wanted a change was that I disliked the kludgy alloy/plastic sandwich sub platter the RP6 came with. Not for specific sonic reasons, purely aesthetic ones. I know that's a little daft when you can't see it in use.
I therefore asked for something that didn't change the Rega sound and maybe enhanced it. His advice was that Delrin doesn't change the character of the sound as much as alloy.
I kept the old bearing and sub platter assembly to swap back in if I ever sell but if i'm honest an upgrade combination of the; Rega Reference EBLT, the Fidelity Design RP6 "Reference" sub-platter and bearing combo, and an acrylic mat, have made my RP6 all the turntable I'm ever going to want.