Rick - I haven't driven mine quite that high yet, but it sounds like me might be wrestling with a similar issue.
I need to go back and refigure my bubble rate. I've been playing with the new Tom Barr ceramic diffuser approach, and things have been interesting. I've got a 46g with KH=7.5, and have been maintaining pH=6.5-.6.6 (though I still have consistent BBA appearing
I started with a DIY reactor , a controller with flaky needle value, CO2 on 24/7 (w/ controller) and my bpm was around 160. Heck, I had the controller, so should the bpm matter? I switched to a low-end ceramic diffuser (Penn-Plax?) and within 30 minutes, all the fish were gasping. I set the bpm down around 120 and things settled down, with the pH running from 6.7-6.8 at lights on to 6.5-6.6 at lights off. A few weeks after that (and about 5 days ago), one evening I switched to an Azoo "medium" diffuser. In the AM, the pH was at 6.6 and thirty minutes after lights on, all were gasping and the pH was down to 6.5. As I was going away for the weekend, I cut the bpm down quite a bit (haven't counted yet, maybe half?) and the pH seems to be hanging in the 6.7-6.9 range now.
So, still in the experimental phase. Take if FWIW. But, a few data points for you.
Personally, I'm kinda with you. Sure, all the bubble sizes are different, and you can't really rely on them for much. BUT, if you're sitting at 200+ bpm, that "feels" like a high number to me.