I purchased the Z36 rotor and pad kit in 2014 for my 2011 RTS. Kit was on sale for around $220 if I remember correctly. Sold the 2011 RTS and the kit sat in the basement. Installed today on my 2014 SE with 50,000 miles, and feels really good so far. The front hardware in the kit did not fit but my old stuff was in good shape. Front pads had about 10,000 miles left. Rears were at the limit and all hardware fit good. I should be good for another 50,000 miles. My RTS had about 80,000 miles on it when sold and had original brake parts. I wonder if there was a change in the hardware from 2011 to 2014 or if the wrong stuff was packed for the front in the kit? My driving habits have changed from mostly highway in the RTS to a lot of around town in the SE.
At 36K miles my 14's rear oem brake pads were completely worn. OEM rotors and calipers are fine. Last weekend I installed the PowerStop Z23 brake pads. These are the pads that come in their tow kit. Everything in the kit fit the rears properly including the stainless sliding plates and the rubber bellows for the caliper pins. Braking power is good but improvements to the rear brakes don't typically make as noticeable difference as changes up front. Time will tell how well they last but in the heavy stop and go driving that I typically do, rear pads are only good for around 35k. Fronts are more like 50k.
I don't think that there are any real changes to the brake hardware for any of the years of the GenI. Heli, if the parts didn't fit then either your kit was improperly packaged or Powerstop has the wrong stuff specced.
Overall, changing out the calipers as part of a tow kit isn't going to gain you anything (unless you really like the color Red!) unless your calipers are actually in need of replacement. I haven't priced out the individual Powerstop parts but I don't see much need in buying the calipers. Just get the rotors and pads unless the "complete" kit isn't much if any more expensive.
Good to know the kit comes from quality material. Switched to ceramic pads at 67K miles, factory rotors were still like glass so kept them. After 85K miles on ceramics, bought the PowerStop package just in case. That was almost 26,000 miles ago. Have been driving now more than 111K miles on same replacement pads (current reading 178K). Still waiting to put the new PowerStops to good use. BTW that Amazon price hasn't come down a penny in almost 2 years:
Just a word of advice; after you bed the brakes in properly, upon stopping, you will hear a low hum when the brakes are applied. If you apply the brakes harder, you may even feel like you hear some grinding noise.
Do not be afraid as those are normal. The cross drills on the rotor has that weird wind-cutting noise, but I can guarantee you that you will feel the brakes bite hard and stop much better than OEM blanks.