I've noticed something similar to what you described. When I was testing some 460 handloads this spring, I hit a plateau. A 200 grain FTX on top of 51.5 grains of H110 gave me a velocity of 2656 fps and a standard deviation of 27, fps while 52.0 grains gave me 2599 fps and a standard deviation of 54 fps. A charge of 52.5 grains gave me similar results. All loads had a tight crimp, so I just accepted that 51.5 grains was the best load for my gun. On the other hand, back when I was testing my very first 460 handloads, I noticed that applying a crimp to cartridges with minimum loads of H110 increased velocity and consistency. These were both revolvers, mind you, not single shots.

My answer to you is to try out a heavy crimp to see if it make a difference, but only if you feel a 300 grain bullet at 1900 fps isn't quite enough power.