You can judge from many of the kayaking videos that are on youtube. Here's one at 10.3'. You can avoid the majority of the waves. I just watched this for the first time; at some point in the video, he was trying to surf; and you can see from toward the end of the video, that he's pretty new to it.
https://www.youtube.com/watch?v=bHugdqneKnk
9.1 feet - very easy; no spray skirt needed:
https://www.youtube.com/watch?v=10CJ8696lYc
12 feet: now it's getting fun!
https://www.youtube.com/watch?v=BrVM1t-u1ow
My son and I ran into these guys in the gorge one afternoon.
If you can see the difference from 10.1 to 11.95, imagine 13.5. Coworker and I went at that level (they close the gorge to kayakers at 14'); as we stood at the put in, I think we were both hoping the other would change his mind. That ride: holy shit. A helmet cam would have been pointing at blue sky one moment, then at a wall of water the next. The waves in a couple of sections were immense. The video doesn't do justice for the roar of that water at 12'. But, I think you get the idea of how fast you can go. I guided a huge propane tank (the size that supplies a house) down the river; I think it was about 11' when I did it; lots of fun. The propane company was going to give me $100 for it; all I had to do was get it out of the gorge. Unfortunately, on the way to my car, I didn't realize I was rolling it through poison ivy. I left it a few hundred yards from the parking lot; never went back to get it. (I would have hauled it on a trailer - way too big for even the van.)