I plan on doing the same thing. My reasons are b/c:
1. I run dumps, so I think, the GT bumper looks better
2. One of my skirts under the door is cracked - I've repaired and repainted the GT skirts once, but the crack came back.
3. I noticed the back piece on the GT skirts gets chipped up really easy, b/c it sticks out. The Cobra skirts don't have any pieces that stick out and I assume they'd hold up longer.