Depends. If the paste is fine, then no, you're not going to see much, if any, improvement. Changing the paste in a laptop is also a major pain in the ass, and not something I'd do unless I was damn sure I needed to.
Is the laptop overheating while sitting in your lap? If so, you're probably blocking exhaust ports. If the laptop can't exhaust the hot air, it's going to overheat no matter what you've got cooling it.