I ran one for 1500km last year and no problems. Can't say what the gains were as I inatalled it when the sled was new. As for C3, they are one of the best aftermarket companies I have dealt with. There were a couple of bolt failures last year so Kevin redesigned it and sent out the upgraded part...
This guy rocks the BRP underwear and has owned alot of sleds and the etec is by FAR the best one. I never owned anything other than carbed sleds so I was real hesitant to snowcheck an etec last year but after riding it all year I don't think I could go back.
1300 km on mine. Still on the original belt and it is still in good shape. No issues whatsoever. Best sled I have ever owned. Towed 2 M8's out this year with it so good luck to everyone that is switching to Cat!