I think some of us don't realize that GT is mainly a technical showcase for Sony and the PlayStation console. One of the best ways to showcase it is through graphics and visuals. Kaz and PD just simply can't go with the ugly graphics of Pcars or Assetto Corsa to be the best simulator and racing experience.
So for every new generation, they have the pressure to push graphics to its limit, while sticking to the 60fps target to the best of their ability. I think it's going to be at least two generations later where graphics will reach the point of satisfaction and Kaz can put in features like dynamic weather and ToD, with 60fps at night without compromise.
But yes, if they want, they could just use GT6 graphics engine, stick with 16 car grid, and have stable dynamic weather and time of day at night, while still looking better than project cars and AC mind you, but PD has the obligation to push the graphics every generation to the point of sacrifice.
So no, I don't think they're lying when they say the ps4 is much easier to develop on. It's just not powerful enough to fulfill everything they want.
This is where you can appreciate PD on how they can just manage to push every console. What they did with GT for every PlayStation generation is unheard of and never matched. GT on ps1 looked great, and the only game to offer some sort of realistic physics, and it also had a mode showcasing 60fps. GT3 for the ps2 was simply wow (sunset, sparks, and heat wave), and GT4 being the only game to offer 1080i hd support. Then GT5 came out with dynamic time and weather, interior view, while outputting at a resolution most ps3 games couldn't do at 1280x1080p. GT6 pushed it further while increasing it with even more pixels and introducing rendering methods never done on a console with adaptive tesselaton.
It will be no different with GT Sport. Visuals will be the priority, then physics, then 60fps. Whether that is a choice by Kaz or Sony or both, it doesn't matter. It's the reality. Again, they could at least stick with ps3 premiums quality but no, they are going super premiums... All at the cost of dynamic weather and time, among other things.