I had my Cush Drive apart when I first got my bike and giving it a general clean. There was no sense of tightness then, but this would have been after much use.
Are new Cush Drive Rubbers so very tight from new? With so much apparent compression, are there Polyurethane alternatives that would retain good shape for much longer?
Finian, I wonder if it is worth checking you have the correct part? If I understand the dynamics correctly, then the compression after use could make them spread and difficult to fit the wheel. Yours are new and are starting 'thin', mine where used and did not appear to have grown in width to make the wheel difficult to fit. Hence it might be worth checking the part (and fitting - are the joiners stopping the veins seating properly?). Just a thought.