#1. If you are not overclocking, probably not.
#2. If you are not overclocking, probably.
#3. Yes. Or when you have thermal problems in a poor case.
#4. Probably - but not to the CPU's full potential.
#5. Unless your after market heatsink has pushpins, yes. If your case has a cutout on the motherboard tray (many do now) under the CPU socket, no.