In my opinion, for what you do with your computer, your system should be way more than adequate for quite some time. I doubt you would have seen or noticed any improvement in your day to day performance with the changes that came with the latest iMacs, except possibly the ac internet speed if you also buy a new router to take advantage of the ac standard (but "n" speeds are pretty fast anyway for most things), or a Fusion/SSD drive.
One easy thing you can do to improve the responsiveness of your system, when/if you feel the need (or want), is to add a SSD drive via USB 3.0, or Thunderbolt adapter and make it your boot drive. This will enhance your user experience quite a bit, as quick boot times and and almost instant program openings are very nice! Only benchmark numbers would show any difference in perceived performance from an internal SATA connected drive, and you would not need to open the iMac to install it. The perceived performance increase is very noticeable from a HDD. But, that is a nice to have change, certainly not essential.
The only way to avoid that feeling of "gee, my machine is now last year's model!" is to commit to upgrading annually with new product announcements. But, that makes no sense from either a technical or, especially, financial standpoint, unless you absolutely need some improved feature (as opposed to want), or you view your computer as a toy over a tool. You will forever be chasing your tail!
Be happy with your purchase, it's a nice fast computer and should serve you well into the future.
. What you think other pre-upgrade iMac owners?
