TBH my lenses aren't so fast really - the fastest I have is the 50mm f/1.8, but I just somehow don't find it useful for storm photography - I use mostly my 10-22mm f/3.5 - 4.5 (actually every photo in this post was taken with this lens) and 70-200mm f/4 USM (for distant storm clouds, which I will show in next post with the rest of my storm photos) and I use the APS-C sensor too ;) But in most cases you don't really need fast lenses for storm photography, especially during the daylight - most of those photos above were taken with aperture of f/8 or smaller (bigger f number). It's good to actually stop the lens down a bit if the lightning conditions allow, because most lenses have best optical quality while stopped down by about 1-2EV - and additionally you are getting a much larger depth of field, so more of the scene is in focus and you don't have to worry about precise focusing that much. For example, the last photo was taken at f/8 - so you don't need to worry about f-number of your lenses ;)

This gets bit more complicated while shooting timelapses though ;)