Having a game on an SSD doesn't make a difference to performance (unless your HDD is heavily fragmented). You will more than likely experience shorter loading times, though, which is always a bonus.
For some, less microstutter and dips in performance when loading/unloading newer or old objects and textures/other resources.
Which in turn takes a hit on FPS.
All that is stored in memory.....
No, it's stored on a hard drive.
It transfers when needed, it can't (or most can't) have everything at full lod loaded. There's a limit on object distance compared to terrain too.
Someone ran a test on a 7200/10k/SSD and there are notable differences post loading, but that's all on a private forum for the ARMA community I'm in.
Streaming from a disk is another matter. I worked on a recent sandbox game as the getting the data off the disk fast enough caused enough problems.
Back to point one, what happens when it struggles? You take performance hits, which are noted in dips and freezes of varying lengths, which in turn hurt FPS.
So bam, that was a long way around to get to the same result.
It's not simply about raw transfer speed, but access time to prevent the above.
http://i.imgur.com/fjMl8.jpg That's during the benchmark and the video below.
http://www.youtube.com/watch?feature=player_embedded&v=y_NVgJooWOQ
So short answer, yeah it helps.
Those were done around release date and the last vid is more recent after many patches and a system upgrade even further, he had other videos from way back doing the same thing.
What are your specs and what settings are you using?
http://forums.overclockers.co.uk/showpost.php?p=24524727&postcount=2576
I'm playing on Ultra with MSAA reduced to 4x, post-process AA, bloom and blur disabled.
I've noticed (at least in the alpha) that lowering the Picture In Picture setting increased fps.
Also having clouds on high and above will hammer fps. They look good but they do cost a fair bit.