Sure, you can use one of the faster powders but you got to be very careful not to double load them. Faster powder will get you in huge trouble with a double load. That kind of load is not nearly as well documented and just like CF load data you just cant use less for some loads. You got to pic the right powder to do it. Light amounts of N110 or 5744 could probably do it but 1500fps is really low.
If that is all the speed you want its simple. Buy the MGM 45cal barrel from Haus of Arms. Its only about $480 shipped. Grab some BH209 and shoot the Lyman 457122 bullet resized to .452 You can buy those from Bullshop for under $50/100 shipped to your door. You only need about 70gr Blackhorn to get there. The MGM barrel fits standard TC ML barrel forearms.
http://www.hausofarms.com/HausofArmsMGM-45-Caliber-Encore-SS-Muzzleloader-Barrels_p_1328.html
