Since no one here has given the definitive answer I needed. I've contacted manufacturers to get at the answers I was looking for.
First, Diablosport. Mike Litsch responded to my email, stating the Intune I1000 has the 3.91 ratio as a selectable option out-of-the-box, and should not require a custom tune. Also, reading their website, it appears that a custom tune would be offered, free of charge, for the Trinity device.
So Diablosport Intune/Trinity WILL DO our 3.91's.