I agree, he has, but still not an indicator of how he will look with us. He usually looks solid for Canada. Of course, it doesn't hurt when your D partner is Drew Doughty and there are three All-Star forwards on the ice at all times. Most guys won't look bad.
Every other time he has looked solid for Team Canada was at a point where he looked just as solid in the NHL. 2014 was best season as a Blue. I don't think you can argue that his game changes significantly from the NHL to international competition. He has looked good on team Canada before because he was a good player, not because he elevated his play significantly.
As for your point about playing with elite guys, I don't think that should worry us. Doughty is better than Petro, but not by a huge margin. He will also be playing with some darn good forwards here in STL. He is generally (at absolute best) the 3rd best player on the ice when he is playing for us (assuming you think he is better than Schwartz/Steen/Stasnty). Obviously we don't have the talent of Team Canada, but the NHL teams we play also have less talent than the competition he has been facing at the World Cup. We won't be asking him to be the focal point. Other teams will still be focused way more on Petro than him and he has some really good forwards to outlet the puck to.
Obviously a preseason tournament isn't a guarantee of his success this year. However, there have been a lot of positives and I think they point more towards his ability than the situation he has been in on Team Canada. His skating looks better than it has in 2 years. If he can keep that up, it will be absolutely huge.