I think someone did some tests while connecting the DR to the PC (via USB) and there wasn't much different in speeds between different classes of SD cards. Now, this does not exactly tell us if the DR can access them faster by itself (instead of the PC), someone will need to write a small disk benchmark app and compile it to run locally on the DR to tell.
Edit: I was wrong. Here is the
original thread (courtesy davidspitzer for the tests). Of course, the comp shows that the higher classes are faster, duh! We still don't know about the DR though.
I suspect that the DR does not have the necessary circuitry to take advantage of those higher class SD cards like a camera does, so it shouldn't make much difference. The random access times don't vary too much between classes, so folder navigation, document switching etc. will remain unaffected, IMHO.
I did some crude (stopwatch) tests pre 1.6 with an Extreme III Sandisk and a regular Sandisk (the blue one), and large documents took about the same time to load (so maybe disk read speeds are not the bottleneck). I didn't test boot up times, but I doubt they would be different. I haven't tried stuff post 1.6.