Stiffen the rear with bar, springs, bump or rebound and in what combination.

Is the instability under brakes only in a straight line, or trail-braking towards the apex - in which case, when does the instability set it - the instant you get onto the brakes, OR as the braking forces build up, or only when you put some steering input into the car as you continue to brake as you turn in ?

If it is one of these 4 that is changing (or all 4), then we can rule out a sticking caliper causing different brake torque across the rear end.

Are your coil-overs independently adjustable for bump and rebound (sorry, have not read the entire thread) or does a single adjuster do both.

Also, when you make the rear "stiff", I'm assuming the car refuses throttle inputs as you attempt to accelerate as you approach/hit/pass the apex which is a tendency to mid-corner/exit oversteer - or is it understeer ?

I might actually go back and read the thread from the start ........... and then comment again tomorrow ....

My 370Z back when I drove it on Laguna Seca was similar setup to yours. Most of interior in car plus a roll bar so it was close to stock weight. Stock under 300 WHPP motor, coilovers, stock wheel widths but with Hoosier DOT R. Was couple seconds faster from your fast lap and thats with huge fuel starve out of T4 ..car was coasting 1/4 of the straight and little out of turn 10. Pumps stopped working that day so I was running 1/2 tank my afternoon sessions. It was banging out 1:44 to 1:45's without thinking. Without fuel starve it would have been in high 1:42 to low 1:43 range. So suspect you have some setup opportunity.