Ok, I played the demo on Big Boss Hard twice, and it isn't significantly harder, certainly doesn't compare to COD4 on Veteran (which is endlessly frustrating).
Crawling everywhere seems to do the trick quite well, and the few times I got spotted I would just run to some alleyway and shoot the enemies/ hide until the alert went away. Ironically I actually found it easier to kill them rather than hide, as the AI is useless in a gunfight, and when I try to play dead they often come up to you and kick you which then leaves you struggling with the controls trying to out CQC them.
I also noticed a few glitches:
1. Near the beginning of the demo I was spotted, and so ran across the street, turned left into an alleyway, turned left again and ran all the way to the end and hid behind an arch. No enemy ventured into sight in the alley, but when I tried to move (in the caution mode) in a crouch walk, they somehow 'saw' me an immediately raised the alert again. Which meant I had to shoot them down again.
2. I got a cardboard box and was hiding in a room with other cardboard boxes, when an enemy walks in and says "what's this doing here" and picks up the box, however at other times I can be in the middle of a street under the box and they won't notice.
3.Sometimes when you're pressed up against the wall, and lean out to( hold L1) fire, your gun refuses to do so, even if you lean in and out again and try to fire again it doesn't, which is annoying. GTA4 also does this at times.
4. You can still evade enemies by running between areas just like the gameboy version, why is this still possible on the PS3? I remember throwing a petrobomb at the feet of 3 militia, and then ran through to the next zone, only to have no alert mode at all (even caution) active.
However the game isn't bad, it's just not the best game ever, as some believe, its very much an 8/10 game if you're a fan of the story.