Ok, a 35W it is
Im also planning to use a SI-97, but not passively. Instead I'll put a nexus or panaflo 92 mm, running at 5V, on it. It will live in a desktop/htpc case so I'll mount the fan to suck air from the HS and duct it to a vent in the top of the case. In this environment it seems better to get the air out of the case as soon as possible. I hope that this, along with the PSU fan will move enough air. Air intake will be "passive" through a lot of holes along the side of the case.
Thanks for all the help