I just finished upgrading a 17" Mid 2009 macbook pro for my mother to an OWC SSD drive.
OWC mentions on their compatibility section of the web site that the 2009 Macbook Pro may downgrade SATA 3 drives to SATA 1, rather than fall back to SATA 2 (which is the fastest the 2009 models go.) I don't know if it was all drives, or only particular firmware of certain SATA 3 SSD drives, but it might be worth a check before you get to invested in making the fusion drive.
Also, just so you are aware, apple considers computers vintage 5 years after the computer has been discontinued. I believe that will be in March 2015 for this model.
Why does it matter? Because after that date, you will not be able to get a genuine battery service from Apple. OWC & eBay offers batteries for sale for a self-install, but to be honest the reviews are mixed when I looked at OWC nupower batteries in these forums, and there are bunch of terrible posts about eBay batteries.
When I was doing research, a genuine apple battery is almost impossible to find (even though the are not discontinued yet.) The only OEM battery I found online was at powerbook medic.com for $200. Apple charges $180 instead, and they do the work.
According to coconut battery, the new battery I received a week ago was manufactured in June 2014, so the battery apple put in wasn't sitting on a shelf for five years.
Anyway, it is a bit off topic for your question, but if you are considering a fusion drive I am assuming you are planning to keep this computer a few more years. I would consider a genuine battery replacement while it is still available, unless you wish to go with a third party battery.
