Strange questions. Depend on what you are plannig to do with them. I have a Sandisk Extreme III (2GB) card, my PC can read/write with 16 mb\s to it, my cam only gets 9-10 out of it, so my guess is the SX100 isnt even able to exhaust the full speed of a "class 6" card. But try it out, i'm interested in it too.
10 Mb\s is fast enough imho, writes RAWs fairly fast, and i can record videos up to quality 95 without problems (though, recording videos with that quality is useless anyways).
As multipartition support was added recently, you could get one of those 8GB SDHC Class 6 Cards, and report how fast the SX100 can write to them - but i doubt it will be faster than 10 mb\s
regards