I've won one tournament with him, sure a smaller one but still had 28 people back in May of last year money prize tourney.
At any rate, there were a few good Cages, one good Cyrax player some stick players. I went thru everyone with Ermac all the way. That was PRE patch/nerf however, now it would no doubt be more difficult.
That aside, Ermac can win if used wisely not a doubt in my mind, the only problem is match ups...if an Ermac is going up against an equally solid KL, Raiden, Reptile etc Ermac's chances aren't as good but that's not to say it's in of the realm of impossible.
Not sure about Evo since that tourney has so many people and a lot of them will use the top guys to get far, but a smaller tourney I think Ermac can win. Look at last years SJ, CD jr won with Rain, I think a tourney of that size or smaller Ermac can win with patience and when played smart.