Upgrading to 12GB will have a significant impact on the iMac's top speed... it just doesn't help much with acceleration (ie system startup times, application/file opening times, etc). I personally thought the jump from [4GB + HDD] -> [12GB + HDD] was bigger than the jump from [12GB + HDD] -> [12GB + SSD].
The SSD upgrade is a pain, especially if this is your first time. I thought the hardest part was disconnecting/reconnecting all the display cables and the 21.5" only leaves a couple inches of clearance to disconnect/reconnect them. Most technically inclined people manage to get it done first try, but if one is as hamfisted/unlucky as I was you'll spend 4+ hours and disassemble/resemble the thing three times before everything is properly reconnected.
The OWC installation kit for 2011 iMacs includes an inline thermal sensor cable, which you need for normal automatic fan control. Try comparing installation kits without a bundled drive + Crucial SSD vs the kit bundled with an OWC 1TB.