Performance gain or loss is relavent to your CPU temperature..... If your CPU is running hot and you put a new heatsink in, you may find slightly faster, more reliable performance increase. And likewise, if you have normal temps and you either get a incompatible heatsink, mount the heatsink wrong, or lack of one, your temps will rise and your computer starts to slow down.
The only performance increase you might see is if your CPU is currently running ridiculously hot (say 60C+) of which a new heatsink will bring it down to safer levels.
Although, sometimes buying a new heatsink can sometimes be at an insurance level if as your heatsink is running at higher temps but not enough to affect performance (Idles temps around 50C) or if its is dying......