After a couple of days research, asking questions on the Prepar3D forum and reading/watching some of the resources I've linked to here, I came up with a solution which satisfied my requirement of a convincing representation of the fast jet terrain hugging flight profile.
Resolution is set to the lowest available: 1024x768. I remember when that was unnatainable even on a top end rig! I have a 3840x2160 85" monitor. So at 1024x768 in full screen mode it does look primitive. But, in a full screen sized windowed mode the NVIDIA card scales it beautifully - to my eye it looks like 3840! Anyway if it looks good - job done!
But I was after high and consistent frame rates, yes. With all the settings reduced to minumum I got a pretty steady 70 FPS with occasional dips.
I set textures at 1024 because - why load textures larger than the screen resolution? A side effect of this was that the F-35 suddenly looked much more impressive - maybe it's textures are 1024? Anyway, bonus!!
Autogen and buildings are super CPU intensive in this sim (I think) so I played with those settings and limited them.
Level of detail radius I wanted to max out to minimise (it doesn't eliminate) visible texture loading. We must bear in mind though I'm flying 50 feet from the scenery and 500 kts, not the 5,000 feet and 100 kts the sim was probably intended for!
The monitor is only 60Hz, so long story short, I opted for the combinations seen here at 30fps, which is above cinema's 24fps (but yes, I know, there is no motion blur), and exactly 1/2 the monitor refresh rate - which I figured would make it easy for the graphics card to double the sim's output and get a perfect sync.
And in an amusing twist, I couldn't get the GPU memory usage above 1.7gb out of 7.1. Dang, it would be nice if the GPU used more memory to cache textures at a distance. With HD textures it gets up to about 5.5gb usage, but that would defeat the purpose of this excercise.
Anyway I'm having a heap of fun with this!
Thanks to all the folk who put extensive research into this subject!