Any empirical evidence that OS & sim in separate SATA III drives matters?

I vowed to try OS & FSX on my single Samsung 840 500Gb SATA III drive, and I can't imagine any better file system performance from a subjective point of view.  I also only have Win7, AIDA64 & FSX & FSX add ons only on this drive and use it for absolutely nothing else.  After all of my add ons I still have over 2/3 of the drive free.  Running artificial drive benchmarks I have the predicted I/O rates.  Unlike in the past when I used OS & FSX on separate drives I no longer have even the slightest hesitations when audio files load or what have you.  So strictly from my anecdotal experience, I can't imagine where I would appreciate performance improvements by separating the two.  Having read about how flash-memory architecture of modern SSD's function in terms of parallelism, there is a measured impact when too many file I/O requests happen simultaneously, but from the article I read the sorts of size and rate of these during flights comes nowhere near the point at which performance starts to fall off.  And yet, I've read some comments re P3D V2 that you 'have' to put the sim on a dedicated drive.  I still tend to think this is recommendation coming from pre SATA III times.

I ask now because I will likely be loading P3D v2 at some point, and it appears I'll be fine for drive space--i.e. will likely manage to stay under 65% full even when it's loaded and add ons are added.

Anyone here have ANY objective evidence that this matters now?  I also really like the idea, and this I think is likely just that it seems cleaner, to keep everything in one tidy package for the sake of simple backups, etc.  This I know though is probably not very material since images of an entire system can be done, but still!