Agent Smith on
www.mcarterbrown.com converted one of his TPXs to shoot first strike rounds. Like you said, there are two things that need to be done to allow it to. 1. Modify the breach opening. 2. Modify the magazines. The breach is the easy part. You just need to dremmel out the opening to fit the D shape of the FSRs. The magazine is more tricky. Again, you have to modify them to accept the D shape of the FSRs. Without having a TPX mag in front of me, I'm not sure how easy this would be. There might not be enough material in the magazine to do this. Though some creative use of some sort of epoxy or the like may get you around this.
Definitely a cool mod project. But in the end is it worth it considering you can pick up a brand new T8.1 that is first strike ready right out of the box for LESS than the TPX? I say no.