Well, I just put one through the whole range I'm capable of. The TC full of juice came in at 1.7 ohms. - On an eGo 3.2v batt, it barely produced vapor. there was some warmth and some flavor, but barely any visible vapor. On a 3.7v eGo clone charged to 4.2, they produce a little more vapor but not much more. With a 5.0v ni-mh batt which runs 4.2v under load with these, it wasn't bad. Pretty good vapor and so-so flavor. On a 5.0v regulated mod (now always producing 5.0v load voltage because I brought my AW IMRs home from work), they make clouds of nice dense, thick, warm vapor and the flavor is right up there with any other device in my arsenal. 5.0v is where I can start to talk about TH too. Nice TH right at 5.0v.
My 6.0v set up is a couple of lifep04 batts. Off the charger they produce pretty close to 7.0v even. However, they can only create 5.0v loaded so it was exactly the same experience as my regulated 5.0v.
7.4... Wow. I took a pair of right-off-the-charger IMR High Drains (not my AWs, but nice ones anyway) and threw them in my maxi rough stack. They read 8.4v at rest. They produced 6.48v loaded. That might have been the most insane vape I've ever had. It was very hot, but the surprise was the flavor was perfect. It didn't scorch or wash out at all. - I have no doubt it would be easy to burn the roof of your mouth with vapor that hot, but the flavor was perfect. The draw was only long enough to get a read from my meter, and it produced a massive lung full of vapor that even fully inhaled completely obscured my monitor when I exhaled. I did that half a dozen times before I'd had enough nicotine to lay off for a while. BTW, the TH was powerful. It was even more powerful than I had imagined it might be.
5.0v is a really nice way to run these, but if you're hardcore enough to want them hotter than that, they won't burn at 6.5 load volts from a 7.4v set up.