Apparently, Kojima used 100% of the PS3's capabilities: http://www.psxextreme.com/ps3-news/3076.html

And later, he [Kojima] said he was disappointed with what the PS3 could only offer: http://forums.maxconsole.net/showthread.php?t=108799

Here's his answer from that last link:

"Game-wise, it's pretty close to the original vision: you sneak into the battlefield and can choose whether to do a stealth game or interfere with the battle more directly. But the graphic, side things like motion-blending and the size of the map, totally was not accomplished to my original vision - to my satisfaction.

"When we first showed the game engine at TGS, the staff were really proud and happy. PS3 was a dream machine, y'know, and we were going to work on this and that - and we had so many ideas. But when we actually started developing the game, we realized there were a lot of restrictions and so it turned out how you see it today. The original vision was to go ten steps further, the reality was just one step, which isn't to say we didn't progress.

I remember saying three years ago that we wanted to create something revolutionary, but in reality we couldn't really do that because of the CPU. We're using the Cell engine to its limit., actually. Please don't get me wrong, I'm not criticizing the PS3 machine, it's just that we weren't really aware of what the full-spec PS3 offered - we were creating something we couldn't entirely see.''

Kojima has alerady said he was miss quoted