CD games installation - HDD Space

No. Some games have installation as an optional step, but if it doesn't then you need to keep the disc and the HDD data.

I guess you're right.
I've noticed some games will install a compressed data in the HDD, some will just copy content from the Disc to the HDD, that's the case in Silent Hill Downpour, I just checked what was actually copied to the internal HDD and removed the files from the Disc folder of this game, a lot of language files and movies in 3D or in lower resolutions were also deleted for other games, they are just sitting there without future use.
To make sure I didn't screw the games up, I tested them, since I was sure I was only removing unneeded and unused stuff, and they worked fine, Silent Hill played intro etc, and the video for the intro was only in the HDD.

I saved over 15 GB across the games I have installed here.

Thanks for the heads up.