I tested the class 6 (Polaroid:Made in Japan) and the class 10 (Panasonic:Made in Japan). Class 6 worked fine. No lagging with HD videos. Fast enough with continuous shots. I could feel slight deference with class 10. Faster responses with continuous shots and HD videos. I agree with what Canon said: class 6 is minimum, and recommend to go for class 10 if you want better performance especaily if you shoot RAWs in a continuous mode.
I don't know...you could try a Class 6 but it might not end up being enough...I just decided to play it safe and get a Class 10. Canon said Class 6 "at least". With quite a bit of emphasis on the "at least"
I had a Transcend 16 GB Class 10 Card in my T2i, and it would take photos, yet not record 1080 HD video, after a few short clips! Canon Tech Sprt told me, that although the card was rated at Class 10, it was dropping down below class 6 at certain times during its use! That the camera was only designed and tested and meant to be used with an ACTUAL CLASS 6 CARD! If you can believe that! I now have a 16 GB Class 10 Lexar Card in it, and it is working fine. (Class 6, is cited in the owners manual.)