Here is some reading for you:Brewing ratios for espresso beveragesDigital espresso or a way to consistency
Basically, many have, for a long time, weighed the dose of ground coffee. So much so, that for most people, its normal to check their dosing once a week or so, and for some, its normal to do every shot. I do it every shot.
But then why, after wieghing the input, would we measure the output in volume? Crema volumes change drastically with different blends and age of coffee.. for how accurate we were measuring the input, we were being pretty inaccurate for the output.
So now, a much more accurate way, is to weigh the shot. Either as its pulling, or immediately after. As far as consistency goes, its the way to do it. You'll notice, many roasters now publish weights for their target brew parameters. Some instead of ounces, some along with ounces.
Also, of note, Ive been watching the live stream of the USBC yesterday and today, and Im noticing more and more competitors disclose to the judges what their dose weight it, and what their overall extraction weight is. Its definitely become the way to be most consistent.