I know that this is an old thread, but i use a Class 10 U3 32GB microSD in my Bravo. My ROM is a customised R.S.K. SE Deodex with DK2nand (this means that only Dalvik is on the Nand, every other component is on SD). Formerly i used a 16GB class 10 microSD and i feel some improvement in the loading speed of the apps and the boot time. Btw., i don't think that it can be improved further, the speed difference is not that much between the two cards as the specifications suggests.Has anyone figured out what the SD card controller in the Desire is capapble of?
BTW, I own a HTC Desire S but that should not make too much of a difference I presume.
So, to answer your question, the built-in card controller may have its maximum somewhere between the Class 10 and U3 specified speeds (10 and 30MB/s).