It's very unlikely to happen that the disc subsystem causes any fps improvements.
You may get blurry textures or even loose some (experiencing black squres then) when they can't be loaded fast enough, but no current disc drives (even SDD ones) generate fps issues or gains.
Even the main blurry factor inside FSX is CPU related and not so much disc dependent.
So the worst thing to happen are blurry textures, the best are sharp ones. No fps relationship here.

So whatever happened on your system, it surely did, but it might be related to something else. Especially such huge gains are .. wonderful.  :)