I don' t know about the 15x more powerful stuff... I' ve never played a X360 or PS3 game that make me think " WOW THAT LOOKS 15 TIMES BETTER THAN XB1/PS2" , if anything I rarely think it' s even twice as better.
The thing is, it never looks 15x better because its not like old graphics were complete shit. What you have to look at is stuff like Lair for the ps3. If you watch the video' s where the game is showing both ground, and air fighting about 2km' s away in game, while rendering mountains, water, smoke, and plenty of building, as well as running AI for all those units, along with physics, and 1080p, it is quite impressive technically speaking. It really does blow the ps2 out of the water in that sense.
The 360 has many titles as well that show how far ahead it is of the xbox.
Physics takes a lot to do well, and the ps2 and xbox had barely any in their games, meanwhile, it has become manditory in next gen titles, and is being fine tuned and focused on by a lot of developers.
Also, we have to remember most of the ps3' s games were developed without the most recent dev tools, and even the 360 only has one game that really pushes the hardware(need I mention the game?). So, even though they are far more powerful, they really haven' t shown off what they can do yet, whereas we have seen the best the ps2 can do, not so much with the xbox, but since it was basically a PC, you can get an idea of what it was capable of.