Steel pistons can cause brake fluid to boil under heavy use. That is why phenolic pistons are used.

Some "steel" pistons are actually phenolic pistons with stainless steel caps, which might make them appear as if they were steel pistons. The phenolic material inside still insulates against heat transfer that leads to the brake fluid boiling and the braking power fading.

Since you are working with lifetime warranty parts from NAPA, here is a post from three years ago that provides a couple of NAPA options, so that you can compare them with what Napa is supplying you with now. Note that the "Severe Duty" option is issued with phenolic pistons.

 

FWIW, when I replaced my front calipers for my 99, Napa provided steel pistons. It may be a year specific thing.

 

Do you guys always replace rotors, or turn them if they are still within spec? I'm gearing up for a brake job... I can't remember the last time I flushed my brake system. I know it's been about 7 years and over 50k miles since the front brakes were done. I've never done my rear brakes, but have inspected them periodically. So, I'm doing brakes all around this time, and flushing! Looking at the z36 kits out there...

 

I will leave the rotors until they need to be turned and replace them at that point. I replace them as they aren't that expense and having all the OEM thickness restored gives piece of mind and hopes of better performance.

FWIW, that power stop stuff isn't that great. I have their 4 wheel kit and recently replaced the pads with Hawk LTS and what a difference/improvement in braking. I was PO'd that I lived with worse braking for nearly 2 years.