On a visit to Vapor Kings, an employee there had an Omega with stacked 18350's running a 1.5 ohm DC carto....
I'd love to see somebody take more than a 2 second drag off of that.
Really, your not gonna get much higher than you're getting now unless you do something crazy like that.
An RBA with a .6 ohm or so coil on a mech mod would get ya there too.
I've ran some 1.5ohm DCs at 15watts with stacked 350s in my Vamo and Zmax. "Uncomfortable" is a word I'd use to describe that experience

It was just too much vapor/nicotine/whatever for me and it felt like my lungs were going to explode. 10-11 watts on a single coil is my preferred setup.
If you are using cartos or normal vaping equipment I think the vast majority of people won't ever want power anywhere near that high. But with Genesis devices and RBAs that high power really is worth it. 15+ watts, in those cases, is glorious, not uncomfortable. So when/if you see people talking about running 20 watts or whatever with their gennys, just know they aren't iron lunged (well, maybe they are) but that 20watts is a lot easier to handle with a genny than a carto/clearo/whatever else.
As far as the volt vs watt limit, I'm curious how this would work on the Vamo. I think it was Phil Busardo's review of the Innokin SVD that showed with stacked batteries, the device was actually putting out 6.3 volts (somewhere around there, it was over 6volts) in order to achieve the wattage setting. I've heard the SVD uses the same chip as the Vamo/Zmax/Anyvape/etc so it would stand to reason (if true) that a vamo would be able to do the same thing.
The 6volt and 15watt limit is a programed in setting, not a real limit as far as I understand it. (could be wrong) The amp limit of the device is what really limits it. I'd be interested to see some tests done with a Vamo, stacked 350s running a 1.5ohm at 6volts. That would be about 24watts, but only 4 amps, so the 5amp limit wouldn't cause an issue, and if it's a 4amp limit then it would just barely make it.
I just don't see why, using stacked batteries, the device wouldn't be able to do this. Stacked batteries gives you (even if the batteries are almost dead) more than the required 6 volts. The amperage isn't too much for the device, so I'm curious what exactly is limiting it. It's not like the boost circuit has to boost a low voltage up to 6 volts. All the device has to do is let the straight battery voltage though, in other words a 100% duty cycle.
I don't think there is any carto/clearo/atty or anything other than a custom coiled RBA or genny that would tolerate that high of power though. You could definitely design a coil that could operate at 20-30 watts and still not burn your juice, but nothing like that is currently sold.