I had my 2.2 5spd up to 105 once. It was still pulling reasonably strong. I think it would've buried the 120mph speedo. My STG1 SS(Cobalt) I rung out 4th gear, which was 141(GPS). I Fully believe it would have hit at least 155, if not the full 160 on the speedo. I don't imagine the HHR would be too much slower top speed wise, but I could see a stock SS struggling at 150.

 

There will be a new race this summer not far from me called the Arkansas Mile. Blytheville Ar has a closed Air Force Base and the runway will be put to good use. We might find out what a 400 hp hhr will do. https://www.usmileracing.com/

 

My SS with turbo upgrade and no other mods still had plenty of pull somewhere over 130.
That was with the speedo. I stopped checking at ~130.
We caught up to traffic and had to shut down.

 

I have been able to get my 2.2 a little past 110 mph. I didn't push it any harder because I was on a highway and didn't even realize I was going that fast. I've never tried to push it since then.