I would have to get the car re-corner balance and there is no way to tell if it would help or not help, I am not going down that road again. I actually had a set of 10K springs too, but I returned it. Also as I said, I have bigger issues to deal with right now. I have to worry about the clutch and the diff. first

I feel for you. Been down this road before. Feels like you're chasing your tail in circles. Once you get it figured out. You go oh crap, that's it.

So where's the formula? We all have the same car so it can't be this difficult to fingers out. At least fortune auto seems to have so sort of formula or they wouldn't ask all those questions then tell you to use "this" spring.

Don't have one. It's something you have to figure out as to what feels best to you. You might be able to drive the wheels off of the setting, but it might feel like crap to me and I couldn't do a thing with it. You might like the car to feel tight. I might like the car a little loose. ;)

That formula is called rwd, 3500lbs, how bad of a ride can you tolerate and customer experiences. You must be kidding to think they tested every car. I do have to say the adjustments do work. One click makes a noticeable difference. I do remember my best lap with just the coilover was when I had the front at full stiff and rear in the middle. The car was at its fastest, but it was so nervous under braking

Suspect if you don't have adjustable rear arms your rear toe settings are out of whack. That can make car feel very nervous .

I'd put most of your handling ills with rear spring rate that is too high front that might not be high enough and toe that is off a bit.

And while you saw marginal improvement with a6 in Autocross if you optimized coilover setup for A6 (-3.5 front and -2.5 rear ) with neutral rear toe and slight toe out up front suspect you'd see couple second improvement on Laguna Seca.

Unless something slipped out of adjustment, my toe should be fine as I already got the car aligned. I meant to say I recall setting the rear shocks softer, I had my best lap of the day with the car like that. THe drawback was that the car was very hard to drive, especially under braking.

So you are at stock ride height with your coilovers? I don't think you can maintain proper toe if car has been lowered without adjustable toe arm? Handling issues under braking to me sounds like toe issue. Or perhaps not enough front spring or front compression dampening

Also check your rear brake calipers for leaks too. My rears were replaced under warranty as one blew at very low miles. That would also impact handling.

What he said .......... if the car is unstable under brakes, every chance it is rear-steering and that can be differential brake torque or change in rear toe as the weight transfers to the front .....

Not sure, car seems to be fine once I stiffen up the rear.

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.