Thanks, appreciate all the input.
I agree that a precision alignment makes a big difference. I went with slight toe in front and back, -1.2 all around, and 5+ on the caster. It made the MX5 handle like I always thought a stock MX5 should.
The car no longer tends to follow seams and expansion joints (well, maybe a little bit on big ones) and I no longer have to correct when going over lane bumps. The car used to have quite a bit of understeer and would snap to oversteer, making things unpredictable. This is now much less pronounced and very predictable, the front still pushes a bit but the back end will break first and it is controllable.
The only thing it didn't fix is how "busy" the car gets when it hits bumps in corners, it actually got worse. I suspect that the reason the car is busy is that it is on the bump stops in the corners/curves, and that it has gotten worse because I now drive faster in the corners, thanks to the alignment. I am actually hoping that this is one thing stiffer bars will help with ... but keep in mind my shocks, etc., have over 60K miles on them so that might have something to do with it as well!
06 Sport, 6MT, Winning Blue / Progress, Koni, FCM, RB sways, RB end links, RPF1 7.5, PSS 215