Did you tried A-B test, same file, slowest card vs. fastest?
If there's no diference it means that there is a buffer or a resampling working on.
In the digital audio one of the most critical aspects is the synchronicity. You had a data that needs to be delivered in exactly some point in time with the lowest delay possible at the moment required. The jitter is the "noise" produced by a-synchronicity. A faster card will help to archive that if you get the data straight forward from the card and convert it to an analogical signal without any other processing in between (the case of the Tera Player that is minimalistic).
It's not about how fast you can start playing a file.
I did. Test were done with two 24/192 FLACs on all cards (except for the 128gb kington class 10, all cards with just these 2 files on root folder only). One is Hotel California, and the other Mozart Violin Concerto No.4 from 2L.no sample page.
Sound quality wise I cannot tell a different. Even the track starting speed or the seeking (FF/RW) there is no noticeable difference. I also included a sandisk micro sdxc 64gb I'm using in my AK120 with stock SD adapter, also cannot tell a different. Even directly comparing the slowest Sony and the fastest sandisk extreme pro I cannot tell.
Of course I'm still buying faster card just simply because I want to have faster write speed when transferring music. The extreme pro does writes much faster :)
(For those who want to try, seems the 901 don't like hotswapping the cards. I got a "SD Card Error. Reboot in 3 seconds" thing after hotswapping in another card. The error persist after reboots. It is only fixed until I boot it up with the earlier card before the swap, shut down, and then put in the new card to boot up again. )