Regarding a speed test of my Sandisk 32 GB Class 4 card with WP Bench Free see my post here http://forum.xda-developers.com/show...&postcount=174
I (also) noticed that my phone slowed down. But I'm not sure if the card is to blame. Previously I had two partitions on the card, a small one for Android and the large one for WP7.5. In December I repartitioned the card with three partitions, two rather small ones for Android (to get newer and faster ROMs working - "NativeSD") and the third one for WP7.8 (my main system). But Windows Phone feels a lot slower now. The reason might be 7.8 being slower or it is caused by the three partitions or maybe even by the card getting slower, I don't know.
But I'm also interested to get a faster WP (again), so I asked other people to return their benchmark results with WP Bench Free in order to have some comparison and maybe a reason to buy a specific new card (32 GB or 64 GB, not smaller). Sadly no one published any result so far.
P.S.: With 7.5 WP Bench Free also returned approx. 4 MB/s, just like now, but the system felt way snappier compared to 7.8.