From what you've listed here you have a kick ass system.
What is the issue you think you need more performance as one could go a few different ways here some easier than others, and cheaper!
Can you describe what you are doing. Are you working with large files (music and/or vids), Gaming, or Coding ?
OK, what do??
I would not swap out the internal HD for a SSD it won't improve things enough. I would look at adding some RAM if you think you are going to work on large projects and/or have a lot of windows open at once.
Next I would look at the way the system is configured. Do you have a lot of open windows when you shutdown which are re-opened when you turn your system on? Do you pre-load apps? While these can be helpful they do tax the system as unlike a dual drive setup the OS is trying to balance what is held in the SSD Vs the HD under the Fusion Drive.
Frankly, if you really want all the apps loaded and/or reload after restarting I would split the Fusion Drive back down to a dual drive setup and then manage where things sit manually. As the OS and Apps would be on the SSD it would be technically faster in loading. Leaving the other stuff on the slower HD only to be accessed when I want it.