Which upgrades to my iMac 2011 will improve performance the most?
You are the best person to answer that. 

Use the Activity monitor and take a look at RAM and HD current usage. 

That will tell you what you need most. Usually the more RAM the better, that in turn also removes some load from the HD, but a fasted SSD could help in the initial step of loading the app in to the RAM.

So if you open and close lost of apps and save lots of stuff a faster SSD will help. If you initially open lots of apps, but then just use them, without need to constantly access the HD (SSD) then RAM size matters. See it this way: RAM is the supper fast (temporary) storage between the CPU and the Hard Drive. Without improvement in HD (SSD) the boot up and the shut down will not be faster, but during the normal usage with large RAM you will benefit.