Turbo has NEVER been considered. Now if they did put a V8 in the H3, and I'm only speculating, the mileage would no be off that much from the I-5, and might be better on the highway end. That is if they add a V8 with active fuel management (aka displacement on demand), as they do in the 6.2 liter in the Escalade. There will not be a 6.2L so don't anyone get all excited about that sentence.
I would imagine, if there was a V8 coming, the rear axle would stay the same, but the front axle would be different. Rear axle is strong, and in those cases where one was replaced and then it went, I would bet under the majority of cases, it was incorrectly installed by the technician. There was some problems with early axles having lapping compound left on the ring/pinion gears from the AAM factory.
As for power, I feel the 3.5L is adequate on the highways and off-road. Then again, for power take the Vette out, for normal driving on the highways take the H3. No, it will not beat the Mustang, but let's face it, most people don't race the engine and want to drag an H3, and vice versa.
If I can keep it at 80 plus on the interstates without straining the engine, then I don't have a problem with a vehicle's performance. Again, if I want to be the quickest off the line, I will take out 700+ HP in a 3000 lb car.