From what I've seen the GT500 gets around 10mpg mixed averaging various YouTube channels, long term reviews and articles. The R would have to make more than 707hp but would also need different camshaft profiles and tuning to make this viable. Ultimately an off road truck that goes 0-60 in 4 seconds is just neat, it doesn't mean that the intended use will improve.
I just want Ford to do a 7.0+L Boss mustang. I would pay ADM and sing praises. Yeah I have a better chance of getting struck by lightning after winning the lotto but we can all dream right?