The evidence is mounting. I have been a competitor in the BPCR game for 30 years. In all the load development of all the lots of powder and many different rifles, have come to think that the much talked about "compression" and "what powder brand likes "how much compression", plays very little in the accuracy of the load.
Quality of powder, bullet choice and fit, bullet hardness in some cases, and primer choice all have much more to do with accuracy than compression. What the compression ends up being is incidental to having the right amount (weight) of powder to hit the accuracy node.
A few days ago, in preparation of shooting an upcoming 300y, 500y, and 600yard mid-range match, I shot my 40-70 SS accuracy load, set up in cases three ways.
Full length cases
Lyman neck-sized cases
and LEE collet die cases (neck sized)
All resulting in a different case volume, and, by extension, different compression.
They all shot the same at 600yards accuracy wise. Interestingly, the LEE sized cases grouped about two minutes higher with all else being the same. This load holds the 10 ring in the vertical. Winds of course stretch out the group left and right.
Fouling control was with a blow tube as the mild spring air was cool enough to not over heat the barrel.