Respectfully, I think you are putting the cart before the horse. It would be advisable to decide about the more expensive components first, and then get the appropriate less expensive components to match, rather than the other way around.
If you happen to have the the eleven speed chain and shifter, which are usually the less expensive pieces in a drivetrain, I would suggest you either give on the idea of the the 12speed derailleur, or start over with a complete 12 speed system.
For what it is worth, and this is totally my opinion here...I used to run 12 speed Eagle with a 10-50 range. Now I run 11 speed Shimano with a 9-46 range (E-thirteen cassette), and I can't tell the difference. I only went with Shimano because I care about the dual release for upshifting. Otherwise, both systems work great.