Maybe it's a bit old for Final Cut Pro, but MS Word shouldn't be taking 10 to 15 seconds to load up. One of my machines is a 2008 MBP that I recently installed a 512GB 840EV into and it's running smooth and fast.
It doesn't matter here, because either sata or pcie ssd always affects the same factors: boot time and app start time.
BTW: Do you really feel the difference between 300 mb/s and 700 mb/s when you launch Safari or iTunes (or something different)?
For a start I'd put the SSD into the HDD bay - I suspect the optical bay connection has negotiateed to only SATAI. That would explain the lack of a substantial speed boost...
Check system information from "About this Mac.."->System Report and look under SATA/SATA Express heading to check the negotiated link speed for the SSD