While I understand the draw of diesel and potential diesel performance, I think one has to ask themselves what they would like to see improved over the current 5.7. Put another way, what's lacking about the current drivetrain?
I personally think the current 5.7 hits all the marks. It's incredibly tractable with gobs of torque, including the all important low end torque. I've experienced many other gas motors where they sacrifice low in torque for high end hp. That's not this motor. Yet, it's incredibly powerful, with hp to spare. I can objectively state this as I've actually towed with my LX570, and I want for nothing torque and power wise. This is on 33's no less, so with stock sized tires, the performance (gearing) is actually even better. The newer 8-speeds, better again. Those going larger tires, can always get their mojo back by regearing if they think the 5.7 is lacking.
I'm likely one of the few who has actually run the vehicle anywhere max GCWR (14400lbs). Recently did a 2300mile trip, with a family of 6 in the car with gear + 7500lb travel trailer in tow, loaded with full tanks. If anything, I was likely just over GCWR. Towed through serious mountain ranges of CA and Oregon, including the grapevine. The 5.7 gasser is smooth and tractable, needed little rpm to truck along on the highway. Grades require spinning the motor more, but there's no significant NVH and the anxiety it brings. Smooth and a low great sounding growl on the contrary. I can always hold highway speed or pass on grades, and I don't find myself needing to let anyone get around me due to a lack of power. Though I often do ease up on grades for efficiency.
I'll agree that efficiency is the one area where a diesel can excel. Increased efficiency to make a tank to longer and farther. Though a gas motor can augmented to go longer and farther too, especially with the aux tanks close readily available. That's where I'll be banking my money. I don't tow and haul enough where a diesel motors efficiency and cost advantage would be a benefit. And oftentimes, is a false cost savings, as up front and maintenance costs can be significantly more, negating any advantage..