Its true that grain size is a big factor but our grain size is consistent, not the density. The question is how does that affect our powder.
In a quick attempt to test it out, I loaded three rounds of 45 colt with 30 grains per volume of three powders under a 220 grain bullet, the exact same case was used in the same chamber every time. (I only have one 45 colt brass case for now haha)
First powder was swiss FFFG with a typical 30 grains of weight per 30 grains volume velocity 912 fps.
Attachment 290715
Second powder was my 30-60 mesh powder but 77 13 -10 this time of brown willow (not my fastest batch) but this one has a density of 1.40 g/cc and 24.2 gr/30 gr volume velocity was 882 fps.
Attachment 290716
Third powder was the same as the second but with a 1.82 g/cc with 30.58 gr/30 volume, velocity was 918 fps,
Attachment 290717
If you can base the efficiency on velocity per grain (weight) since the bullet weight is the same:
swiss : 30.4
2nd: 36.4
3rd: 30.02
Now obviously this isn't a full blown large scale conclusive test with several shots and several bullets weights and barrel lengths, but I think it gives a birds eye view of the effect of density on efficiency and fouling.
edit: not a 220 grain bullet, this was a 230gr RN that I typically use for 45 ACP