I mentioned the temperature peaks because there is some people that misses them, just because happens in a specific point of the game, like in the game The Last Of Us, there are 2 points of the game (where appears water) that increases the temperature a lot more than the rest of the game, is like a temperature peak, but mantained for long time (as long as you stay in that area, close to the water)
Thats the max temperature your PS3 is going to reach (worst scenario posible), is important to know that value to have the things a bit under control
For someone like you that is having around 55ºC while playing you should not matter much, because that peaks are around 4º or 5º more than the rest of the game
Lets say... 95% of the game the PS3 is going to be at 55ºC and at that points is going to reach 60ºC as much
Thats ok, you should not worry unless you reach 80ºC or so... and you are far away from it
And yeah, what you mentioned is important, at the time of doing this "stress temperature tests" is important to have the PS3 turned on and play the game for lets say... 50 minutes at least