Here's some seat of the pants thinking.
A 38-40 has a nominal water capacity of 36 grains as shown above, 39 grains as reported here:
http://kwk.us/cases.html. SAAMI max pressure is 14,000 psi.
That revolver, the New Model Ruger Blackhawk, is also chambered in .45 Colt which has a case capacity of about 41-42 grains of water. Several sources report max "Ruger only" .45 Colt loads of up to 30,000 psi. I forget whether Brian Pierce or John Linebaugh came up with that number based on the relative thickness of the chamber walls for a .44 Magnum vs. a .45 Colt.
The .38-40 case is .467" OD just forward of the rim, the .45 Colt is .480", so there's actually a wee bit more steel in the .38-40 chamber wall at its thinnest point, then the walls get thicker from there. But you have to load to the strength of the thinnest portion, not the thickest.
Hornady shows the .45 Colt with similar case capacity albeit bigger bore (more bullet base area for pushing) throwing a 300 grain jacketed bullet up to 1250 fps or a 250 grain jacketed bullet to 1300.
As you noted, Hornady shows a 200 grain jacketed bullet reaching 1050 fps. They don't say what pressure that load is, they only caution that it's for modern revolvers of similar strength to the Blackhawk. I doubt they're even getting into 38 Special +P territory but that's just wild ass speculation.
Anyway, I wouldn't say to load the .38-40 to 30,000 psi (how would you know?) but just looking at what that revolver is capable of, I would say that with your 215 grain cast bullet at the same 1050 fps you'd still have a
very large safety margin.
This is all just Mad Scientist thinking on an overcast and windy afternoon...
