I don't see how powder speed would make much difference in that case, it might be worth a try - but first off do the longer seated bullets actually shoot much more accurately in that rifle? I have the same circumstance with a couple of Savage rifles - one of them in particular (a .204 Ruger) has a very long throat. I have done some pretty extensive testing of different powders, seating depths, etc and I never actually saw any better accuracy with the longer seated bullets. Kind of surprised me, but that rifle just does not seem to care what the seating depth is. Sure simplifies things as I was also planning to load 2 different depths for that rifle. I have another Savage in 7mm-08 that actually shoots better groups with the bullet seated at mag length than it does .020 off the lands.
It probably has more to do with physical properties of the chamber/throat and bullet ogive than propellant speed, but I might be wrong.