We can try to think the main usage of the microSD..
for example: 1. Taking pictures or video recording. 4~5MB/S writing speed is quite enough. 2. Browsing or watching video/photos from microSD. 8~10MB/S is also quite enough. 3. Launching application from microSD. The application is often less than 5MB...
So, there's no need to persuit real high speed microSD. Since most microSD are single channel. Cannot compare the speed with regualr SD cards(Dual-channel).
unlike SSD controller, the SD controller won't focus on small package writing. You won't have much chance to write small file to the microSD when using PDA Phone.