Danne, I tested only with the SanDisk Extreme Pro (95 MB/s), since I do not have a slower card at the moment. Your latest hardcoded build that allows up to 42 fps works very well if I do not switch modes too often. As soon as I switch from 5x-magnification to Movie Crop Mode for example, the 100D freezes, I have to pull battery out, restart camera, reload modules, run the overclock test and only then I can continue working.
As far as safety is concerned, I cannot comment on that but I never noticed a problem. Even after a crash, all my previously recorded clips remained perfectly intact on the card. I never lost a clip, unless the crash happened during recording. This happened only once or twice and I cannot reproduce it but the reason may have been a battery level close to running out of power where supply voltage may fluctuate and drop below critical level for short periods of time during recording.
Overall, I am very satisfied with the overclocking feature. Again, I think, it's revolutionary!