That's not quite what I have. My current (temporary) set up is 2015 5k iMac with 2013 27" iMac in TDM. Just until some work on my house is done at which point the 2013 iMac will be moved to a different room.
It's not bad, but retina/non-retina isn't perfect. Being side by side accentuates the lower pixel density of the non-retina display. El Capitan handles things well, but win 10 vm in latest VMware fusion seems not to cope well.
I would seriously consider a 4K display, eg from Dell that will likely cost less than a Thunderbolt Display. Just my two pennies.