I was doing some BBF yesterday and played around with the speed and thought I'd pass along what I found. I weigh in at 195lbs and 34mph was very comfortable for me on the 5' lead off the boom. The other thing I noticed was that when I really concentrated on bending my knees, flexing my ankles and arching my back so that the handle was at hip level, it was one smooth ride. Don't know if any of this will help it's just a few observations I never thought about before.
Thought I'd drop you a quick note of follow-up regarding the back footin discussion. From the sounds of it you are a bit more consistent than me these days and you are getting a good amount of time in in your feet. When I was consistent 2 years ago, I found that a slower speed helped me to grip the water in a sense and the faster I went things became squirrly/slippery. The other thing I found was that where I was looking had a big impact on my stability backwards, so I just kept my head up and looking straight back at all times.
With that said, you have some really good footers out there and I am sure they can offer far better advice than I can but it's good to hear you are doing do well with it. If you have not invested in a neck roll, I highly advise it as it makes the next day a little more doable!!!