<snip> I am only interested in a product that will produce more vapour / more of a hit. Am I right in thinking going from a tornado / riva510 to a 6v machine would achieve that ? <snip>
OK, so lifetime is not an issue. The other posters above have done a fantastic job of commenting on various mods. I'll add 2 cents worth to the quoted question. Since it is performance that's the issue, you may want true 3.7 volts rather than the 3.2 ish from the Riva. Also, you could go to 5 v (or even 6v), and be worse off if you don't factor in the atomizer ohm rating in knowing your "sweet spot".
It's about the watts:
"Standard" 3.0 ohm atty on 3.2 volt device is (3.2 x 3.2 / 3) = 3.5 watts
2.5 ohm atty on 3.2 volt device is (3.2 x 3.2 / 2.5) 4.1 watts
LR atty (1.5 ohms) on a 3.2 volt device is (3.2 x 3.2 / 1.5) = 6.8 watts
LR atty (1.5 ohms) on a 3.6 volt device is (3.6 x 3.6 / 1.5) = 8.64 watts.
"Standard" 2.5 ohm atty at 5 volts = 10 watts
"Standard" 3.0 ohm atty at 5 volts = 8.33 watts
HV atties vary = can be like 6 ohms or so (not an expert, I don't use em now).
Now, some people (including myself) like the 3.6 volts and find that the LR atties get a little "hot" and "burn" through the juice and the flavor. Others find some juices work better at higher voltages. I have a "5 volt" direct PT (more like 4.5 volts) that is ok, but can be a little "hot" with 2.5 ohm atties.
The point is that it is the COMBO that makes it work. You are correct that more voltage = more "kick" right out of the gate. But you can still be unhappy until you get the combo that works for you and your e-juice. As mentioned above, I'd seriously see if you could try a 5v PT (a direct PT? or one that specifically says 5v) to see if you like it. 6 volts is tough to play with. The higher the voltage, the more specific you have to be on the ohms (since the voltage is squared in the above calc).
So, for me anyway:
1) Standard 3.2 volt stuff - meh.
2) The 3.6/3.7 volt u-solder box mod (about $8.00 for parts not incl batts and charger) is a good vape with lots of flavor. 14500 batteries are $5.00 each. Charger $10.00.
3) The 5 V PT is less flavor, more "kick/fast start". More juice too. Needs a 2.0 amp wall wart.
4) I'm currently building a 5v dual 14500 mod - jury still out on the "true 5 v" experience. Just adds a few bucks to the box-mod costs - already have batts and charger from #2 above.
5) 6+ v - IDK yet.
The cool part is I use the same 14500 batteries at 3.6 and 5 volts. Same charger. Could use LR atties at 3.6 without worrying about frying MOSFETs too.
Others swear by variable voltage mods so they can "dial in" the voltage to suit their needs.
Also, as I'm sure you know, PG adds vapor, and PGA can add TH if you want it at any voltage.
Good luck!