I think it makes a lot of sense for them to do this.
The Powerboost only makes 41hp. And maybe they reliably squeeze another 50 out of the Ecoboost for an even 500hp. So 541hp at most, but they'll have to call it 500hp + 41hp, because typically the max system power will be nowhere close to that because the torque curves on the gas and electric motors won't match.
Stick the blown V8 in 15% of them and charge 20k-30k extra for it. They'll be priced right up against the Ram, but without making the Raptor generally unaffordable to us proles.
Personally I'd still buy the "lame little V6". I do miss the V8 sound of my 6.2L but I'd rather save the $30k and buy a base model Mustang GT that would still be faster than the TRX or V8 Raptor, and more fun to drive on-road (which is the only place you can use 700hp with less than two feet of suspension travel).