I recently just took apart my case. Mind you, it hasn't been opened in almost 4 years so there was tons of dust everywhere in it. It was crazy. Anyway, I bought compressed air and cleaned it all and there was no FPS boost. If there was, it was a very slight and unnoticeable boost.
It would be a good thing though to clean out your computer to avoid it from over-heating. In my case, I guess it was the design or fan that made it so that it never over-heated.