..from some time back, when the HDD was first offered. At least one of the rifles tested (the PCP) was more or less well modded.
A whole lot of it is how much vel. variation you can stand, and evidently i can't stand a whole bunch. generally, will hold co2 to the same standards as HPA or PCP guns and not accept more than a 30- 35fps drop in vel.
Can download/print the graph in your post, figure out where 33fps would be, draw a line for each plot, and find the number of shots at that level of drop off. HDD is worth about 10%. That's better than it looks, is like getting an employee discount on co2.