It's either replace the whole flex pipe and catalytic converter

GM Cars Cobalt ion HHR G5 CK Info Below Walker 54579 Catalytic Converter Gaskets | eBay

Or get one of these welded in

Semi Direct Fit Exhaust Flange Repair Flex Pipe Replacement Kit w Gasket FX2061 | eBay

Thanks! That was my next question!

Or, go to eBay and search for "hhr exhaust repair":
hhr exhaust repair | eBay

Ah, if you look, those are eBay links.

Yeah, I saved those links in my ebay thing. I am going to see what the exhaust shop will charge to just replace the flex pipe if thats what it is. Personally, I would like to replace the whole exhaust with a bolt on unit but I have to be budgetly with this car. I am a motorhead :D

I went to the 2 1/4 inch 2.4 exhaust and opened up the pipes from the stock 2.2 1 3/4 inch exhaust. There are some Magnaflow systems out there for sale, from members here on the site!

A couple of the units on eBay are cut and bolt/weld.

OP did you ever figure out what was causing your problem? I started experiencing the same thing with my 06 this week. In the morning it drives fine with the A/C off. After getting on the highway and running the A/C (especially in the afternoon) it seems like the car falls flat. I can be doing 70 (speed limit by me), need to pass someone and have the pedal in the floor and nothing happens, like the tranny just fell asleep. Eventually it will half-ass rev just a bit and gain some speed. Really getting frustrated.

Did you check for a leaking flex pipe/exhaust?

Hey guys,

So, here's whats been going on with my situation. Thanks for asking.
Not sure if if I ever really fixed anything yet, but a little better. Since my last post, I went out and unhooked the battery for awhile and actually, for whatever reason that seems to have done the most good thus far. I suppose it base lined the ecm and did a better job re-learning. Whatever. It is a little better but I'm still not satisfied because it still seems lacking in power with a/c on. Also gas mileage seems low. I've been averaging 18 mpg in town but much better on the road. the dic reads like 6 mpg on take-off. What the hell?
I also put the jack stands under the car and started it up and got underneath and didn't find the exhaust leak sound I hear, not the flex pipe. Now I think it may be an exhaust manifold leak. Don't know how much that will affect the performance though. It is covered under the wheelz warranty I have on it if it is the manifold. So anyway, the only thing I know to do next is clean the MAF and throttle body whilst un-hooking the battery again :D Other than that, maybe my cat is clogged, or maybe the a/c compressor is going bad and dragging it down. idk. Hope this helps.