I have to say I've found the complete opposite. COL has made all the difference, powder change just makes the group go in different places (within reason). Sure if you have the wrong COL then powder changes might result in a nice group, but not consistently.
Just my 2p
And if its raining then this and part 2 is worth watching. But skip the bits about concentricity and come back when you have 1/2 moa groups.