Just to be clear, Baribal is talking about 2 3/4 Drams of black powder NOT 2 3/4 Dram "equivalent" of nitro powder. The difference is that nitro (modern smokeless) powder is more explosive. Consequently, it burns faster, even when the blaine is very coarse. The faster burn rate translates into more pressure because it builds up more quickly.
The answer to your question is "No. There is no way using Dram equivalence, Shot size, Shot load, and/or maximum velocity to calculate peak pressure".
What the Gamebore loads (and others like them) do is to mix retardants into the nitro powder to deliberately cause the powder to ignite and burn more slowly, thus reducing the peak pressures down into the 8,500 psi range. Still, I had to look long and hard to find the pressures that could be expected from these shells. Even Gamebore understands that they mustn't guarantee the maximum pressures of these shells because they could randomly produce a 'hot' load or two at any time. Used in an "iffy" gun, they could result in damage or injuries.
For this reason, shooters who purchase and shoot vintage guns are wise to inform themselves of the facts, the risks, and best practices. No different than any other firearm, you need to know what you're doing. Equally like any other firearm, they can be used perfectly safely provided basic safety measures are observed.
Check your Elsie for dings, cracks, pits, rust and other corrosion. "Ring" the barrels to see if the rib is coming loose or problems exist that can't be inspected visually. Keep her clean and lightly lubricated. Check often for obstructions in the barrels while in the field (leaves, snow, etc can get in the barrels while stopped for a rest, for example). You've probably heard all this before. Just keep in mind that, the older the gun, the more important these become.
Oh, and enjoy your time in the uplands and your vintage Elsie. No greater pleasure can be had with your pants on!
ETA:
My comparison of the two powders is intended to be a discussion about commercially available shells. If it becomes a discussion about one type of powder versus another, anything is possible. For example, tests with black powder have shown that, under the right conditions, pressures as high as 100,000 psi are possible. While this is factual, it doesn't contribute to a discussion on appropriate loads for vintage or other shotguns.
