As far as I know, many of the higher class cards have been produced by biasing performance to continuous read/write speeds, for use in camcorders/cameras etc so they can record ultra HD or burst capture etc. HOWEVER, for use in a phone, which needs greater random read/write access they can suffer compared to a regular class 4 which is a better balanced card. There is a lot of this going on in the HTC Mozart pages here on XDA, where I've been spending time (had a Mozart, ROM'd it and gave it to my girlfriend, she loves it).
Many people have tried different 32gb/16gb cards, different classes from 2-10 and the results have been surprising. WP7 seems to be unusually fussy about the cards that will actually work with it, and the only card guaranteed to work everytime is SANDISK which says a lot about the quality, and people have been finding that performance with a Class 4 SanDisk card is better than performance with a supposed Class 10 card by another manufacturer such as Integral, Transcend etc or even Samsung and Kingston.
In the end I bought a 32gb SanDisk Class 4 off Amazon, for a lot more than they are on eBay at around £28, apparently there are loads of near-fakes on eBay (only way I could guarantee it was genuine- the chinese are using genuine 4gb SanDisk cards and then re-branding and re-coding them as 32gb, the card thinks it's a 32gb, reports that it's a 32gb card but obviously runs out of room and has errors as soon as you try to put stuff on it).