PTs are super.
Power (amount of heat, warmth, vapor) is from Ohm's law, voltage squared divided by resistance.
510/eGo/Riva family have a regulator that keeps the output voltage at about 3.1V. Typical KR808D-1 output voltage is 3.7 - 3.8V. A passthrough will typically actually deliver somewhere in the 4.7 - 5V range (if it has an inline battery all bets are off -- it may actually only be outputting, for instance, 3.1V, if it's an eGo sit-and-go type PT).
Take a 2.7 ohm atomizer or cartomizer and put it on the three devices. The power is:
510: 3.6W
808: 5.1W
PT: 8.2W
The 510 runs noticeably cooler than the 808, and delivers less than half the power of the PT.
(I really like 2.6 - 2.7 ohm on an 808, but that's too hot for a PT, prefer 3 - 3.3 ohms on them.)