In 2011 the swaybar should be 25 mm in diameter and the end links should be 9.8 inches

 

In looking at my Carfax report on my previous 2011 2LT, the first entry says it was registered as a rental, but the second entry when it was sold at auction with 16,000 miles says it was a manufacturer vehicle, so it may have been that instead of a rental.

 

That does not matter. Some rental cars were leased from GM. The codes in the glove box tell what is installed.
Don't worry, just get FE3 parts; Pretty rare to find FE1 parts and FE5 are for the SS. FE1 and FE3 are interchangeable, the valving on the shock absorbers is the only difference I know of. Some claim they can tell the difference, I can't.

If you don't believe the sway bar size, measure it; wrap a piece of string around it then measure the length of the string. The links are the "10 inch"/9.8 inch (because they are really metric 249mm). the parts books are wrong stemming from misinterpretation of part numbers. My personal experience was that the 12 inch links caused the stops on the sway bar to move and permit the sway bar to hit the LCAs. Makes no difference in handling or ride.

Many parts books claim some parts "fits all" that is usually incorrect if you want FE5, but true for FE1-FE3.

The answer to the question "is the strut different in rentals" is NO. Your car came out of the factory with whatever the code in the glove box says.

 

Most rental cars are buy back returns, so they manufacturer sells it to the rental car company then buys it back usually in 24 months, then either stocks the used car lot at dealerships, or if the mileage is high they auction them off.

 

There is quite a difference between the FE1 shock and the FE3 shock. The FE1 is nothing special and they were worn out at 50K miles in both my 2008 & 2011 LS's. My 2008 FE3 had Sachs monotubes, looked identical to the SS Sachs monotubes but with a different part number, which I speculate was just different valving.

I'm still hoping to figure out if FE1 struts were different than FE3 struts. I just haven't put the effort in yet. Kinda expect they were different. In any event, what I have learned is that parts supplier listings are vitually useless for sorting this out.

 

Both of my HHRs came with FE1 and replaced with FE3, including rear springs. I could not tell the difference, I used to drive 200-500 miles every day.
As I have been saying for a long time: there are very few parts, if any made as FE1. As far as shocks & struts are concerned "fits" is all most vendors care about, i.e. lengths (min & max) and mountings

 

I have a 2011 HHR LT2. Mine says 2LT. I want the softer ride too. Got new struts/shocks, tires and still really firm ride. Hate the jolt I get when go over a bump or crack in the road.. My next try might have to get the 16 wheel/tire combo to put more rubber on road for soft bounce not shocking hard bounce..

 

You have a 2LT, that is a trim level. Suspension is either FE1 or FE3.
Are you following the tire pressure placard on the driver door frame? Many tire stores and owners always put 32PSI in the tires because the label on the tires says "max inflation 32 PSI"; they don't see the "max" part. Most HHRs are placarded for 28 PSI, you could go a couple PSI lower for a softer ride.
Wait until the shocks and springs wear out for a softer ride.

 

It's completely plausible that the rental car had different from standard, because it was a fleet order, even if it carried the FE3 RPO. GM could accommodate if they wanted something different. In that case it might carry yet another RPO code that no one ever heard of.

Do you still have the VIN for the old one? You could check with a GM parts department for the part number on both cars and compare.

Try running your tire pressure on the soft side. Also, how sure are you that they're the exact same tire? Pirelli P4 and P7 come in different load versions, SL single ply sidewall, or XL double ply sidewall, or something like that. Can make quite a difference.

And by all means, do update the dampeners if they're in doubt. But at least some of the FE3's had Sachs monotubes in the rear, a little stiff but tend to last. I'm still trying to figure out what difference if any was in the struts on FE3.

 

Look at your RPO code sticker in the glove box, FE1 or FE3 in the 2LT