If the fan RPMs are from a software internal fan control app, you could speed up the fans a little more.
I run software internal fans controls, also. I run the fans around between 1400 and 1600 RPMs.
You could, also, purchase a small high speed desktop fan that blows cooler air onto the back of the iMac, too!
Also, have you tried to vacuum out all of the different venting holes and ports around the iMac?
To do this, you need to unplug everything off of the iMac, disconnect from power, lay the iMac screen down on a soft towel. Using either a small or conventional canister vacuum with a crevice tool attach to the vacuum hose, lightly vacuum out the top vents all of the ports, the port on the back of the iMac in the middle of the support stand, all of the connections ports, DVD slot, SD card slot and if you can along the bottom where the speakers and RAM is installed. Do this lightly so as not to scratch the iMac's finish.
If you own a smaller vacuum appliance or small vacuum designed for use with electronics, use that to vacuum out all of the dust,,lint and dirt from all of the iMac's outside openings.