I sell the exact same parts, just for $50 less, compress one and measure it....its not .620 at bind unless they have a different idea of bind. I would also never run them over about .550" as more lift than that needs more seat pressure and installed height. At say .575", you should be at 1.80 installed with 140#, at least thats what we do here, I dont know know what they do.
They are correct in the 300#@.550 and the no machining though. I tend to disagree with their numbers on a few things including price. We only sell a few of their products like EFI AG and 850 TB's.