Stage III Exhaust???

Eric-the-Bastard here with his first round of stupid questions...

I've decided to be cheap and get a used Elise (I was originally looking at a new SC). Seems to be a good selection of 2005s out there. I don't really care for mods, but it seems everybody has put some personal touch on their car.

I see a lot of ads listing Stage II exhaust and the one I'm looking at later tonight has a Stage III exhaust.

Do these refer to just muffler systems or manifold/header all the way back?
My experience has been that virtually any aftermarket solution offering improved HP usually results in an undesirable compromise in the torque curve and a naturally aspirated Elise seems to be on the ragged edge of that already.

Anything anybody can tell me about aftermarket exhaust systems in general for the Elise?

Cheers,
EtB
Sylmar, CA