er you should know the answer better. only you know the total size of your 'programs' and 'fps games' (especially games. bf4 alone is about 56+ gb?). if it's solely for os and some productivity suites, yes it should be fine.

normally yes but it depends on the type of activities (reads or writes. usually availability of free space affects writes more). when the ssd space is getting filled up, there will be a point that the ssd controller needs to do extra work in order to sustain writes performance (e.g. moving data around to free up space for further writes. the time needed to complete the same task become longer, hence you feel the slowdown of performance). this is why as a practice we purposely set over provisioning up in a ssd so that there will be some reserved space for the controller to work with. note that some drives already have additional OP done so you don't need to do that.

if the activities involves more reads than writes (os booting, app loading, etc.), it should be no problem even if the drive is 90% full.
again, for general use, you might not experience that.

Most of my stuff store at D except those important program like my productivity like Ms Office, Adobe PS/AI stuff.. Only took 40%. No games at all.

this should be a good pick.