All dynos produce varying numbers and are not directly comparable.
Absolutely the case. There is a multitude of input parameters as well as output correction factors to be considered - even when everyone is trying to play fair. Using the same dyno consistently (assuming none of the set-up parameters are changed inbetween) is a reasonable way to assess the impact of modifications. Comparing dyno charts from one dyno to another is really not very precise. Tests that are "same day, same time, same dyno" can be very revealing on differences between different cars and set-ups. However, so can venues that tend to mitigate driver skill. Not ideal, but the standing start mile venues align pretty well with HP. We have only one - potentially imperfect fast run with your car which puts it just a hair SLOWER than a 19 psi Gen 2 Whipple dyno'd locally at a mere 695 RWHP. Based on that datapoint, I would say your car was in the same ballpark and, if further, I knew that you were down 2 psi of boost, I would say it was a good result.
So, Ed, you should feel "special". All of this "discussion" is because your numbers are just so far out of the norm that they generate a lot of interest, to say the least. The good news is that this is often the case with genuine break-throughs where initially there are a lot of doubters, stone throwers, and non-believers.... but they are eventually put to rest!
I just say to you as straightforwardly as I can..... that given our experience with the GT and a multitude of GT engine configurations and forced induction methods, rarely do we see something that is such a leap-frog ahead of everything else. I'll be the first to eat crow when and if we have some objective substantiation of these claims.... followed immediately by a multitude of questions for you and John about just what the heck is going on inside this engine! Until then, I remain quite skeptical - and none of that is a reflection on you. If my best friend comes to me saying - 1) massaged GT engine + 2) 17 psi boost + 3) 91 Octane fuel = 850 RWHP, then my first reaction is to say that something is amiss with the instrumentation. So, please turn on the oven as I don't like crow at all but it is probably worse when cold!