I would guess you wont see real world differences with class 2 vs class 6/10..other than video recording/playback. but when transferring files there is no way a class 2 can compete with higher rated cards.
and I wish I knew about the SDXC cards working... might have to order one and send the 32 back.
I can't definitively say that the phone slows transfers since I haven't seen a reliable source report that.... I have yet to try the class 10 card myself since it hasn't arrived. I only know what has happened with the cards I do have.
From what I read in regards to card class and phone use, the majority consensus seems to be that in order to get "true 720" video you need to have a card rated higher than class 2.. considering this phone does 1080 recording I didn't want to be cheap.. for an extra $20 I got a card from a reliable manufacturer and source that was the highest class and capacity available, it wasn't a hard choice for me.
I transfered a couple 3.5gb .mkvs over to the class 4 card I currently have in the phone and it kept timing out or the transfer times would jump up to 30+ minutes and then fail nearly every time. This card works flawlessly in other applications so it might very well be the phone slowing down these transfers. I tried with a variety of other class 2 and 4 cards with similar results. Plugging any of the tested cards into the card reader in my pc transfers the files without a hitch.
Lifehacker did a write-up awhile back.
http://lifehacker.com/5804661/does-it-matter-what-class-micro-sd-card-i-have-in-my-android-phone
I've also read that it is a TERRIBLE idea to use the phone as USB storage as the speeds are horrendous. I can vouch for this as my Transcend 32GB performs faster in the usb adapter than through the phone. Why? I have no idea