I own a 2006 srt design and from everything ive read and learned myself, is that they're basically an srt8 without the engine upgrade. And possibly a stock cold air intake, mine draws air from behind the drivers side fog light basically. But thats about all ive come up with. So i have srt8 seats in mine, 20" srt rims, the front bumper is the same as the srt8 and i have the small srt8 spoiler on the trunk. I've also seen some specs that say they have like 8 more hp but i cant confirm that. Ive even tried to find production #'s for the srt design but no luck so far. No matter though, cause i love my 300c srt design lol favorite car ive ever owned.
Oh and the srt design didnt get the brembo brakes and i dont think it had any suspension upgrades either. I dont know about your srt design but mine has every option available for that year, im pretty sure. Jbl premium sound, sun roof, heated seats and mirrors with auto dim. My side mirrors even tilt down in reverse to get a better view. Hell mine even has headlight washers lol