Agreed.
That grille is used for the intake vent for the PSU. I recently opened my case to fit and upgrade, and there was a big circular pattern of dust on the grille from my PSU fan drawing it in.
I think it would be possible, but it would be a very tight fit. The problem is the space required above the radiator for the water hoses. The space is about 120x250mm, so those hoses need to fit in a 10mm space at the top of the rad, so only some radiators would fit. The other problem would be where to put the pump. I think the only space available would be at the back of the case above the PSU and behind the optical drive, and even then you would have to use a very small one, like a Alphacool Laing. You would have to be very careful about hose routing, and you may have to make a hole in the top of the case for filling. I think it would be possible, but it would make quite a project, rather than a simple DIY install that could be done on a normal sized case.
The spec is correct.
I am using the stock Intel cooler that came with my Q6600, which I am running at stock speeds.
I get around 57 C when idle, and it goes up to about 75-80 C under load. I would upgrade my cooler, but as others have said, there is only 75mm of headroom between the CPU and the PSU.