GRC, we at first had the same feeling/thought.
Concerning sealing to a round pipe: when squeezing the rivet, the exhaust pipe will get flat around and under the rivet. We haven't seen any sealing problems yet. BTW DON'T use aluminium rivets! The pictures in the website show a toy-tool and I think alu rivets. We use M8 steel rivets and need a big and strong tool....
Protruding into the pipe.
Before using these rivets, on our own testbike we have used 2 bungs (M18x1.5) per header for testing purposes. 1 used for a wideband sensor for a selfadjusting system, 1 used for the AFR sensor for the dyno. All 4 wideband protruded into the pipe. We could only measure 2 HP loss!
So only 2 small rivets, instead of 4 big widebandsensors, won't have a lot of influence.
We still think this rivet system has more advantages than disadvantages.
But maybe someone comes up with an even better solution.... we are willing to learn....