Jbuck: Indeed I do add 10% Beef Kidney Fat to my Elk and Deer burger.
The Beef kidney Fat is (according to my long time wild game butcher friend!) the sweetest of the beef fats.
Anyway myself and the rest of the VarmintFamily really enjoy this 10% mix.
I have tried Elk burger many times with no added fat and much prefer the added variety.
Be sure and hand trim all the sinew and white tissue from your trimmings before you take the meat to the butcher for adding the 10% Beef Kidney Fat - don't trust the butcher shop personnel to take the time to do this - this method ensures great burger!
Try it - you'll like it.
Hold into the wind
VarmintGuy