Not all People might not be looking to get every last hp but I imagine they at least want 80% when they spend $500-1000 swapping from a 2 bbl to 4 bbl.
Difference between 650 and 750/850 might not be much but from the formulas 390-450 cfm for a stock ish 360 to what 600-650 cfm most people would put on they would want that hp gain. But picking for a mild combo really ain't that hard I bet most would agree 600-650 cfm for small block maybe some would say smaller for a 273 and 750 for a big block. It's all the other cases where this stuff becomes an issue especially when wanting a good balance of max performance and driveability.
As for the 3 ratings, my main point was there should be 3 stock, performance and race the vacuum numbers I quickly picked weren't the point, the ones if they did do that should work with that formula and help get people a little closer in the ballpark.
