I don't know it's still difficult to judge Golovkin. His best win is over Lemieux who is at best exciting. His resume is packed with B- level fighters and until he goes in with an A it's impossible to tell how good he is. His defence is the weakest part of his game. He gets hit with the type of shots he will want to avoid against Canelo. If he was a more elusive kind of fighter, he'd be more of a problem for Canelo, but he isn't and Canelo probably has the power to trouble him.
That said, i think Mayweather could have stopped Canelo if he really put his foot down, and Golovkin is accurate and carries a much bigger dig. He cuts the ring off well, although i don't think he will necessarily have to go looking for Canelo, there might well be a point where Canelo ends up on the back foot, and once Golvkin switches into stalker mode, he normally ends the fight.
I do think people under estimate Canelo's ability. He is wrongly seen as a less technical fighter than he really is. That's partly because he's been matched against a couple of guys who are designed to make you look bad in Lara and Trout and the fights have ended up being close. It wasn't the power that caught up with Khan on Saturday night, it was the timing. Canelo threw that same shot about half a dozen times, getting closer and closer each time. It was Khan's failure to spot it that caused the problem. Credit to Canelo for spotting the opening and for noticing that the opening was frequently there for him.
So far, Canelo has mixed in better company. Even a weary Cotto is better than anything Golovkin has been in with, and by some distance. As is Lara and Trout. Even Kirkland and Angulo present more danger than anything Golovkin has faced (with the exception of Lemieux). He has impressively beaten a lot of solid fighters, but he still shy of a break out victory in my opinion. His keenness to face smaller men (his promoter was talking about a potential Khan fight) is something that bothers me given he is a fairly big Middleweight. He's had very limited opposition at Middleweight for he last 2 years or so, and i'm surprised he hasn't sought better challenges up at 168. Most truly great fighters move out of their weight class for greater challenges. As good as he has been, there's still something there telling me to not get too carried away by him. He's largely been in with solid, out-gunnable guys who will try and trade with him. He's not been in with anybody who has a speed advantage, or who has an elusiveness about him. People pick holes in Canelo, but he does at least have a variety of opposition. Golovkin has been fed a steady stream of cannon fodder, with the sole aim of creating an aura - which has worked.
Canelo will be his first real test, and looking at Canelo's history with the scorecards, Golovkin will need a KO to get anything. He's not winning on points, so providing Canelo can deal with his power, Golovkin isn't winning the fight.