That all make sense since the 510s run around 2.2-2.3 ohms. A slightly higher resistance on a 306 would make it run a little better at 6v without getting so hot. It should be closer to a "5v (10watt) sweet spot" than a 901 and 801 at 3.7v as well. In other words, cooler than a 510, leaving more flavor, and warmer than the other big guys in the field that run around 3.4-3.5ohms.
The stock 510 batteries have been shown to not run at a full 3.7v. That's why lots of people say the 510 seems so much better on 3.7v mods or even a kr808 batt with adapter.