yea im a bit suspisous of the test results myself, personally the only times ive installed them on friends cars, we welded tubes through them, to make it a faux muffler. It shouldn't be that hard to test, a box with a fan and a 2.5 inch outlet, move it near your computer, grab an extra case fan and rig it to the output, and check the rpm increase, more increase (or decrease depending on wich way the fan is pointed), means more air is coming out. I realize this is nowhere near as accurate as it could be but ****, I'd not need to modify the muffler to do the test, duct tape will suffice, I'll try and remember to pick one up next time im in town, I can just return it later since im not doing it any harm.
EDIT, actually this could be a good bit of fun to try, next time I'm in town I'll grab a bunch of different mufflers, dynamax, cherry bomb, and i guess a flowmaster, the local pep boys always has all three on the shelve, also my neighbor has a small equipment company, Ringo hill farms, they make equipment and trailers, I'll ask him to borrow a tractor muffler, i wonder if the lack of fiberglass would make any flow difference. end cost should be next to nothing, because I will return the mufflers anyway. couple cents worth of duct tape and a bit of time, should be fun.