Typically flow bench values are given for a pressure drop of 28 in H2O. To convert flow figures from a different pressure drop to 28 in H2O use the formula above.
Example: You have flow figures of 152 cfm at 10 in H2O. What if the same head was flowed at 28 in H2O?
I liked the fact that the Thumper heads put up competitive flow #'s with the stock valve sizes. This keeps the port velocity up (more low end power/torque) and then you also don't have to worry about possible piston-valve clearance issues.
I found the Thumpers preserved all of my low-end performance while adding power across the rest of the power band. I'm happy with them (Mustang dyno numbers below are with Thumper/Tmoss stocker, stock cam, stock upper, 1.7 rr's and exhaust.