I have tried this experiment on a calm day and also on a windy day as well. I shoot a Steyr lg110 and my pellet of choice is the 8.4 jsb. I shot the heavy jsb at 45 yards and the group was larger than what i usually get with the 8.4s on a calm day but the group was not horrendous either.
Also i noticed it was an extra half mildot drop on my scope as well when i am usually a mildot drop at that distance with my zero.
On the windy day i did not see much improvement due to the slightly less accuracy at 45 yards and the effect of the wind on the pellet made it less predictable than the 8.4s as well and also adding the extra hold over to the equation just was not worth it with my rifle.
Just a quick note power levels were changed to try and keep it a level playing field for both pellets.