You can try someone like Ekotuning BTW they don't list 5E's. Still give them a call or try the guy that DLH talks about in this video. In the description he gives you the phone # and an email for a guy that tunes tractors named Cody Miller.
I will tell you that JD as well as most all manufacturers locks their computers up tighter than a nun rocking a chastity belt because they don't want people buying 55 HP tractors for $4K less than comparable 75 HP tractors and then converting them to the same machines. Still these tractor handhelds with custom tune written on them aren't cheap but the claim better HP and MPG's.