The VTF-2 mk5 and PB-1000 Pro are similar, yes, but they are not the same. They do have differing performance targets, and they have somewhat different strengths. Those who want to get into the nitty-gritty can compare the measurements from the reviews to see exactly how they differ. Something else to keep in mind is that for burst measurements, I used REW to measure the SVS subs and Don Keele's CEA-2010 testing software to measure the Hsu subs. These programs get very close measurements but not identical, so I would call a +/-1dB difference a wash. In other words, don't take every tenth of a decibel in those measurements as the absolute performance difference between the two subs.
As for distortion, the more complex the program material, the less audible any distortion quantity would be, if it is audible at all. At a high level, you might be able to perceive the difference in a sine wave, but in something like an explosion or an avalanche, forget it. I really couldn't tell you how the distortion quantities would mark a difference between those subs, but for most drive levels and most content types that people listen to, I wouldn't think it would be audible.