For me, there are two reasons to spend money on an aftermarket heatsink:

1) Stock one allows the component to overheat. 54C isn't exactly good, but it's no reason to spend $40+ on another part. Then again, if you're hitting 70C+ on load, yes you need to fix that.

2) The stock one is too noisy. Then I'd look at getting a huge chunk o' metal, and strategically place several large, slow speed fans.