I currently use a 250gb Crucial M4 as my OS drive in my 2600k system. Do you think there would be any noticeable real world difference if I upgraded my OS drive to an 850 EVO or 850 Pro or another current SSD?
I went from an M4 to an 850 Pro and I'd really have to try to really notice the difference anywhere that's not read/write benchmark. I can notice it a bit more on the start up/shut down times, they're a bit faster I notice but how often are you doing that?
it depends. Its a solid drive but not great for steady state and a few other aspects. If you do not notice any hanging than your fine. Only real reason to upgrade is large capacity, which directly affects steady state. Going NVMe is really the only major real world difference unless you are badly affected by steady state but if you were you would have noticed stalls and hangs already. If anything get a new drive for larger cap like a 1TB for 270.