Correct me if I am wrong but you are basically doing mechanically what the new "unicorn/wonder" powders do by delaying/spreading out the area under the pressure curve. So even with the same or lower peak pressure, the total energy pushing on the bullet is more which equals more speed. Doing that with longer burn/slower power, etc. needs a longer barrel or you just throw flames. I expect the barrel length varies depending on the cartridge and powder. Great idea BTW.
Not sure if you can feel it or not but would bet the recoil is slower, more of a push than the sharp hit.