Some information about an extra SSD drive in MacBook Pro
Crucial. They've got a 4.5-star average review out of 810 reviews on Amazon. Further, their 128GB model only sets you back $104.97.
OptiBay is the only option I've heard of for this type of modification, and I have yet to read anything negative about it.
I've never heard any such rumor, and I've actually gone and installed the 256GB variant of that same Crucial drive I linked in my own 2011 17" MacBook Pro with nothing but outstanding results.
This post at MacRumors would go along with your idea to use an enclosure from OptiBay:



  First, I ordered a kit from Optibay, you can google their site. The
  base kit comes with the "frame" for the SSD to fit into your Macbook
  Pro, as well as a USB enclosure for a slot load SuperDrive. The cost
  of the kit is $99, a bit pricey, but it was well worth it and came
  with the enclosure.


5.As far as performance is concerned, you're gonna find the most bang for your buck by installing the operating system on the SSD for sure, as well as any apps that you find yourself using most often.