An ES of 40 is in the ball park for that load.

It really depends a LOT on the different types of powders used
to get that high of a fps out of that load. I have had some
powers at that speed with a ES of 26 to a high of 123 but this
does not mean that either is not accurate.
I had a fast power at just 15 ES and it was all over the place.

In my steel J frame I have two loads that average 851 and 861
velocity but my most accurate load in this weapon is at only 822
fps that will group and shoot at the same spot every time I shoot it.

Some J frames with the factory FBI ammo only clock 800 fps
which leads to.......... do you really need the extra fps in a load?