The second reply has a pdf attached that walks you through the trouble shoot for your code. Compare that to what you already know.
http://www.renntech.org/forums/topic/36978-code-p0446/
Boxsters/Porsche's can be sensitive. Ideally you need a mechanic that knows Boxster's intimately, not a generic foreign car mechanic. If your mechanic is using metaphors, perhaps they should be teaching English instead.
Regarding your WP X/O. Do you know if your mechanic used the right antifreeze to refill your system? Porsche antifreeze is $35.00/gal and should be mixed 50/50 with distilled water. Your system holds +/-6 gallons. Mixing Porsche fluid with something generic from the parts store can cause gelling and overheating.