my father took his gtp to the shop and have them replace the front (was cracked), rear sways and trailing arm. the sways and trailing arm came off a 04 monte carlo ss btw and i was able to take some pic for comparison. i'm not 100% sure, but i think the one on the mc and impala are the same as gmpp. i have gmpp trailing arm on my monte carlo and from the look of both sets, they seem to have the same thickness and look. as far as the front and rear sways on the gtp and monte, the monte feel a little heavier. as you can see the trailing arms are way different. on the gp looks like pontiac just cut a sheet of metal piece and bend them like a horse water traw. first pic is gtp. second is monte carlo.

I'm 99% sure the 04-05 Impala SS already has GMPP sways. Not sure about the trailing arms.

i'm pretty sure the trailing arm are cause it would make no sense if the sways are and the trailing are not. like i mention before the trailing arm on my mc are gmpp and the one on the pic are my factory one. they both look the same to me.

I think you're right that the trailing arms at least perform the same, if they're not the same exact part. I seem to remember now that 04-05 SS owners only really need to add front and rear STBs.