Okay, after further discussions with Motorola support, here's the summary: "So, it's a bit more complicated than what I wrote earlier. The Class 10 card will work at rated speed for video recording. But if you were loading the SD card with media from your PC, it would transfer at the Class 6 speed of about 8 MB/s."
Personally, I'm not concerned about transfer speeds to my phone from my computer. It's taking HD video and pictures where I want the faster write speeds, and it sounds like the Razr supports that.