I measured the output on my VTube with a voltmeter from 4.0v to 6.0v in .5v increments and it was within, .01v of the readout at each point in time. What I didn't do was measure it under load I guess. How would you do that? With an atty on it?
Regardless, my wife has a Provari V2 and just last night she handed it to me to take a hit. The best I can describe it is providing a really 'full' vape. I thought 'holy cow that's good' and asked her what she had it set on. 4.5v she said. I run my VTube at 5.0v and it didn't hit like that. Don't get me wrong, it hits good, very satisfying but just not the same. I thought maybe it was just my carto, I seem to destroy them, so I swapped tanks and although my tank didn't hit quite as good as her tank on the Provari it was still a noticeable improvement over how it hits on the VTube. I tried her tank on the VTube and experienced the now expected degradation of vape. Juice will play a factor of course. I'm hitting a ~20mg 63/37 mix right now and she's vaping a ~10mg 70/30, both DIY. Still though, by trying each on each device I come away with the inescapable conclusion that the Provari just plain hits better... I just can't definitively point out why. Maybe it is this drop under load you speak of.
Still though, I'd recommend the VTube to anyone 7 days a week. For the cost it's hard to beat, at least with the 30% off promotion. If I didn't have the wife's Provari to compare it to then I'd think it was the best thing around (much as I did with the Twist when I got it, which now seems a laughable device sadly).
All that being said, my Provari should be here next week ;-)