Dont believe it at all.
The guy that claims something like that, (an improvement in fps by defrag) is talking nonsense. He probably was flying in another area, different time or weather, different airplane attitude (level-descent consumes more fps, climbing doesnt), different Ai traffic in surroundings...or many other reasons.

A very badly fragmented drive would give more loading times when starting the game, or pauses when changing bgl regions. But wont affect fps once the scenery-airplane is loaded into ram-rom memory.

It is true that all aviation forums are plagued by claims saying you should defrag and how it improved fps...but, same thing happen with medicine claims, "take this and you´ll feel better", and when you do, you think it was the medicine and not the owns body recovery. :)