This is what i have found out with my bk2, I was shooting a 10.8gr arrow at 31 inches with a 100 gr tip.
Now I am shooting a 8.2 gr arrow at 28 inches with a 100 grain tip.
Neither has been chronographed as I do not own one (yet).
At 10 yrds the 10.8 gr arrow sunk nearly as deep as the 8.2 gr arrow giving the 10.8 gr arrow nearly 3 inches more penitration in the same target.
This test was done with field points not broadheads.
The biggest difference was the sound made when the 8.2 gr arrow hit the target making a loud thud and a lot more vibration in the arrow shaft.
I repeatted this two shot test about 4 times and each time the result was the same.