Of the options that you suggest, I would think that considering the hassle-factor and performance, that replacing your optical drive with the 840 pro would be the best option.
However, if you have a late 2011 Macbook Pro then don't you have Thunderbolt? And if so, I would suggest a RAID 0 Thunderbolt external drive; Lacie, G-RAID, and others are making both SSD and 7200rpm external RAID 0 options that are excellent.
Thanks for the response!
Yes, my Macbook indeed has a Thunberbolt and although a TB RAID sounds pretty sweet, for my needs I find the FW800 external as a media+projects drive to be fairly ok - at least I don't think it's seriously bottlenecking my setup - so at this point it would be more of a luxury than necessity that I can't afford.
As for hassle in regards of replacing SSD - I'm looking to reinstall my system anyhow (to start the new year with fresh start, so to speak ) so that wouldn't be much of an issue.
I guess in essence my question is whether it would be a good idea to install the Samsung SSD as the main drive and run OS/applactions AND the cache from it, or would I see a noticeable improvement if the SSD was used solely for cache and OS was on a different SSD?
You may hear from others with varying opinions, but I would think that going with the 840 pro as boot/os and cache would work fine for your situation.
Then, go ahead and repurpose the 128GB apple SSD to the optical bay. You can tune/tweak where you put what files after you have all the hardware configured to find the "optimum" setup.