I have the 150 K2 on my 24 nano and it works great, but heat is sometimes an issue, but not a bad enough issue to cause me to have to take any action. On 12g, it might be worse though.
A side note... a lot of sites let you upgrade the bulb for a few more bucks. If you can, upgrade the bulb to the Hamilton, XM, etc rather than the JBJ.
The JBJ bulbs have a track record of premature envelope rupture (thus ruining the bulb). Mine failed in that exact way after 3 months, JBJ replaced it, but if it had done it a week later, it would have been out of warranty. Even still, I was without my lighting for a week and had to set up a backup pendant I had, but many people have no backup...