You are correct. There's not a lot of info available on that set-up...anywhere.
There is a fellow who is a member who works for Barry Grant. Maybe he will chime in and get you some answers. Or maybe you want to PM, email, or call him. His screen name is "Tech @ BG".
I chose to do a 67 vette tri-power because I wanted something different and something that would give me considerably similar performance to a single 4 bbl set-up.
Yes, I am pretty sure I could squeek out a few more HP over 6,000 RPM with a modern 4bbl set up, but I am more interested in what is delivered from idle to 4000 RPM (which is where my motor will spend 98% of it's time).
Did you read the following post? Lots of good info in there.
Why that over the BG? Primarily because of total CFM. The BG set-up is too small for a 511 BBC. It flows 750 CFM. The vette tri-power flows 1050 CFM (adjusted CFM flow rate for 2 bbl's is about 930 CFM for that set-up).
That should be your primary decision maker, "Is it well matched to your motor CFM?"
Second decision is what is more important to you?
Do you want something different?
Street or strip?
Hope all that helps. In the end it's your car. You should do what you want with it.