I don't see why you want to run these newer versions of OS X on minimum RAM specs.
Every iteration of OS X has needed more CPU, GPU and RAM resources.
Apple's minimum RAM requirement is just that! The bare minimum for the OS to function.
It is by no means optimal. Generally/usually speaking OS X, by itself, needs at least 2 to 3 times this amount of RAM to run smoothly and reliably on its own.
What exactly are you using your iMac for?
If you are running an Internet browser with Adobe Flash to watch any type of streaming video, this alone will eat up lots of CPU, GPU and RAM resources, alone.
If you are multitasking by also accessing email at the same time and using the finder to look at files/data, 2 GBs of RAM is barely enough to accomplish these simple tasks.
Do you use iTunes?
Do you play music in the background with iTunes running and other apps also running?
This takes even more RAM to do this.
You say your iMac runs fine on the minimum RAM spec.
Fine is a relative term. You might be happier with the speed of your iMac if you add extra RAM
RAM is cheap enough these days to add to Macs.
You have a MacBook with an SSD installed. SSDs, while great, right now are not cost effective in terms of cost per GB.
If you can afford a MacBook with an SSD, you can't spring some extra cash to add RAM to your iMac to make its performance better?
Any one here can tell you that the more RAM, the better many things on your iMac will run and function.