I just now saw this post. This is interesting as it would also depend quite a lot of which calibers you are discussing. As I have some 45acp which I've never noticed enough muzzle flash to even think about it. Then there's my 357 Coonan which produces a great ball of fire just shooting soft nose rounds! Granted HP's out if that I've are tremendous
or my 22tcm which is similar to the Coonan! Mini balls of fire but still the same issue.
Now 9mm varies quite a bit but I have older green boxes Remington jhp's that I used to stock up on, you know the 100 round boxes the sold at Wally World that ammo has fairly consistent yet mild muzzle flash. I use a lot of Hornady critical defense that hardly has any muzzle flash.
As far as an actual test to measure the muzzle flash of various rounds, I would need more equipment than I currently have to video all of the rounds and compare. I honestly don't have the equipment or the time to do that. So all I can offer is what I've noticed from my limited range times recently:-(