The Bart is going to be stronger than either the Tornado or the GG slim, and the Tornado will be about the same as the GG, but last longer. The only math that really matters here is
watts = volts*volts/ohms
The watts are the power (heat generated). Volts is the potential the heating element (coil) experiences, and is a function of the battery type (these are all 3.7V Li-ion, but there are 3.0V Li-Fe batts), as well as the circuitry between the batt and the coil, like swiches, resisters, etc. The Tornado coil feels 3.1V, not the full 3.7V. So does a regular 510 batt, as well as the 510 mega batt.
The ohms is the resistance of the coil, and depends on the atty itself. Regular 510 attys are about 2.5 ohms. Low resistance attys are about 1.8 ohms. High voltage attys are high resistance, and are about 4.5 ohms or more. The DSE510 "washer-type" 510 attys are about 3.0 ohms.
For my Bart, which is a true 3.7V PV, using 2.5 ohm attys,
power = 3.7V*3.7V/2.5 ohms = 5.5 watts
Your Tornado with the same atty,
power = 3.1V*3.1V/2.5 ohms = 3.7 watts
This is actually the same as with a regular 510 batt, but it lasts much longer.
Once you think in terms of watts, you will be able to fine tune not only what you are looking for, but also what you have. I happen to love 5.5W. Its stronger than regular/mega batts, and attys stay healthier longer because the somewhat higher heat keeps them unclogged more...that's a biggie for me since I vape only VG juices.
What would happen if you put on an LR atty to your tornado? watts = 3.1*3.1/1.8 = 5.2 watts. This would make the tornado almost as strong as the Bart, or other 14500 battery box mods (14500 is a 3.7V battery the same size as a AA, 10440 is the same size as a AAA). The LR atty will of course drain your batt faster.
Your 5V PT with a 2.5 ohm 510 atty:
watts = 5*5/2.5 = 10 watts. Thats too much for me, but some people love it.
Your 5V PT with HV attys:
watts = 5*5/4.5 =5.6 watts. That combo would be a winner for me.
The 14500 mods are often "box mods" and look like the Bart, but as far as I know the Bart is the only USB charge/vape box mod PV. There are sleak 14500 mods too, like the famous one is the Screwdriver. Totally Wicked makes it, but others do too for cheaper. I looks like it sounds, big metal almost elongated-egg handle, and the atty/cart sticking up. I don't think it has a master switch for pockets though. There are other sleek ones, but I don't know any with a switch like the box mods. You might get lucky though.
But as you can see,
1. If you just try out other attys (LR, HV, DSE, Nhaler.com has a range), you have the means right now to tweak your PVs nicely, and find out where your sweet spot is in terms of watts.
2. High voltage is not necessarily high wattage...it depends on the atty resistance.
If I am conservative I can vape all day on my Bart on a single 14500 batt. If I am heavily vaping, maybe 5 hours. We can get an idea of use-time differences between the Bart (900 mAh) and the Tornado (650 mAh). If we call these two numbers the Duraton, D:
D = Amps * hours = (volts/ohms)*hours
D/Amps = hours of service (continually pressed button, given the atty doesn't fry)
For the Bart, and given that 900 mAh = 0.9 Ah:
hours = D*ohms/volts = .9Ah*2.5ohm/3.7V = 0.62 hours
Each button pressing time length takes that much off the total 0.62 hours.
For the Tornado:
hours = 0.65Ah*2.5ohms/3.1V = 0.52 hours. So given the same vaping, I will get a bit more time out of my Bart than a Tornado.
Your Tornado with an LR atty:
hours = 0.65*1.8/3.1 = 0.36 hours
More watts with the LR, but shorter batt time.
Study all this, and you will be in great shape to really understand PVs, and get you to primo vaping for not a lot of money very soon.
PM me if you have more questions.
Good luck, good vaping!
Kurt