I was looking at getting 32GB RAM for my late 2012 27"iMac i5. Did you notice any speed increase?
There seems to be a misconception that RAM improves speed. The general thinking seems to be, the more you put in, the faster it will go.
RAM does not improve speed.
In traditional applications in the past, when an application on a computer ran out of RAM to store information, it would move information from existing RAM and page it onto another media like a hard disk drive (HDD). This storage area on the HDD was known to be a paging file. HDD transfers information slower to/from RAM than if the information was keep entirely in RAM.
It is this movement of information to/from RAM to paging file that slows down the computer.
Once there is sufficient amounts of RAM where it no longer requires to page information to a HDD, the system runs at optimum speed and will not perform faster.
A 8GB iMac can run just as fast as a 32GB iMac if the applications do not demand more than 8GB usage. Adding extra RAM will not hurt your system.
Knowing this, only install the RAM that you need to run your applications without any slow down.
I hope this helps