here is a pic of the bad thermostat that was in my HHR as you can see it appears that the rubber inner seal pulled back
Also it appears that the new thermostat has been updated or at least it is coming from a different manufacturer
when tested side by side the new design tstat opens at 180 like the old one but for each degree it goes up there is much more room for coolant to flow through

 

Have been looking over the overheating issues on the forum and my HHRss is having some of the same symptoms. It is an 08 model, but I found the fan not coming on. Of course the wiring schematics I've found don't show the proper info. I did go straight to the fan and it works. I'm supposing it could be the unit attached to the fan housing (module?) or something else. $300. Is a little steep, but could this be the problem? Running it with a switch when needed for now.

 

Yes, but I don't think $300 for the part is correct. Use the Search tool there is lots of stuff specific to the SS fan module. The wiring diagrams are wrong.