When you say big HP plans, are you plans to still street the car, or more towards Drag Racing?
Since you already have a manual car, I would recommend a Tremec T-56 Magnum XL. The entire transmission plus install kit is $3495 from D&D Performance
If your plans for big HP include mostly drag racing, you're correct, the automatic is the way to go. I've not seen any write-ups on converting to an automatic on the 2011+ Mustangs, but it's pretty sad that Ford's decision to continue using the inferior MT82 6-speed manual is driving Mustang owners to purchase automatics, such as me.
My 2011 5.0 GT transmission lasted for just over 20k miles before needing replacement, and the brand new transmission they put in the car never was as good as the original. I traded that car in on an automatic.