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Jim, I installed Maximum Motorsports plates as a lot of us on the forum have. On my Laguna Seca I was able to dial in -2.0 degrees of camber without hogging out the holes for the camber bolts (the camber is not adjustable on the Boss as you probably know). Some guys have been able to dial in -2.4 degrees. Overall, it definitely makes a difference with turn in performance of the car as well as more stable tire wear. Vorshag makes plates for the Boss also, I believe ther are pricier and can't vouch for if their plates are more durable than the ones from MM. I can say the MM plates are made very well and they have a reputation for the bearings lasting quite a while.
I used the M&M caster camber plates. The thing I liked was being able to add more caster. I also put in the White Line A arm rear bushings which also allow some extra caster. The thing I like about the caster is more self centering more feel. By replacing both it stiffen up all the rubber normally found in the front of the GT. I don't know if the Boss has the same A arm bushing and the rubber in the strut mount.
I had MM plates installed before my first track day.
Yes, they help with turn in--the car just wants to turn.
But the real reason we need them is controlling tread wear and keeping the tire flat in corners for better traction. Without them, you'll scuff off the outside 1/3 of the tread well before the inside part wears out.
Of course, a tire running flat in the corner with a compressed suspension puts more rubber on the road for better traction.