I'm glad to hear you are going to get a round ball rifle.
High velocity for a small bore, ie: .40, I found I had to develop with the super slippery Lehigh Valley Lube. That velocity I chronographed was 2,260fps using 65.0gr. 3F powder. In order to match that accuracy with 2F, I had to increase the powder charge to 75gr. of GOEX 2F. That load not only duplicated the 3F's accuracy, but the velocity as well. Note, I was using a .398" ball in a Goodoien barrel, with a .0235" thick patch.
Now, the same speeds (2,260fps) and accuracy was obtained in my GM .45 barrel, same lube, .445" ball and using 75.0gr. 3f and 85.0gr. 2F, both GOEX powder, with a 10 ounce denim patch, I measured at .0225" with my calipers, compressed as hard as I could.
2,260fps is certainly high velocity for a patched round ball. With velocity comes pressure. Equal velocity = Equal pressure
My .69 also produces what I call high velocity - 1,550fps with a 482gr. round ball, .030" patch. The charge required is 165gr. GOEX 2F. The pressure generated is very much LESS than the small bore loads listed above, but that is only part of the story - the rest if the story here, is called recoil, being many times more than the .40 or .45 produce. The .69 is also much more accurate, especially at 100 to 300yards.