I believe your machine had 8 Gb memory standard. Depending on what sort of things you do, adding memory might improve performance by allowing more programs / data to be cached in memory. I suspect that a casual user won't see a lot of improvement, but it's hard to predict. With El Cap and Sierra, anecdotally the big jump is 4 Gb (or less) to 8 Gb, going higher doesn't get you as much in casual usage.
The stock CPU is probably a Haswell core i5. If you were to upgrade the CPU you would probably see a performance improvement more or less directly related to clock speed. The 27 inch shipped with a minimum 3.2 Ghz CPU; I'm not sure what your options are, but it's unlikely that you could realize more than 10% improvement even on CPU bound tasks. Just leave the processor alone.
I suspect that your best upgrade would be to replace the hard disk (assuming you have one) with an SSD. This can have a very noticeable effect on perceived performance. If you have a Fusion drive, going pure-SSD might help or might not, depending on your usage patterns.
The next iMac iteration was the late 2015 model, so you wouldn't have seen anything different direct from Apple.