High efficency means reduced need for cooling, so Platinum is always something to strive for. Considering the small heatsinks needed in the passive Seasonic X-460FL 460W ATX psu, I honestly don't believe it impossible to have a passively cooled 450W SFX psu. Yes, an SFX psu has quite a bit less volume than an ATX, but going Platinum should fix the problem. Not saying it would be easy, or cheap, but possible. Here's a shot of the sinks in the X-460FL, and note that it is "only" Gold certified, so they could possibly be even smaller: That said, I 'd prefer higher power output instead of fully passive cooling, as long as the fan is quiet.