Whereas I will fairly regularly kite a twintip, surfboard, and foil in a single session. With our experiences so different, I can't help but think there's some missing variables here in our descriptions: different wind measuring, different wind variability, different kite styles, different riding styles, different wing capabilities, different speed preferences, differences in what feels powered, etc. It would be ideal to be able to kite the same conditions on the same day for a proper comparison, but that's not likely going to happen!
Unfortunately where I kite there aren't many foilers, so I am rarely on the water with another. I haven't yet seen the tiny kite action in person -- one day I'll be interested to see how that works, especially if it's day that someone is on a 3m while I am on an 8m, and we're both able to ride the way we want to.
Back to topic: so far for me, high end on foil has been high 20s and a 6m. I could have been on my 5m, but would be sketchy on the lulls in between. Into the 30s, not sure I'd be tempted at this point to foil -- maybe it's our wind, too gusty/holey/surgey, like riding a mechanical bull sometimes.