for speedo calcs…
http://www.miata.net/garage/tirecalc.html
With the lower cornering stiffnes of the high sidewalls the car will feel more lathargic (it will require higher slip angles for lateral acceleration), however since you are applying it front and rear the understeer characteristics of the car will reamin the same.
Maybe run them at 32-33 psi to bring back a “sporty” feel.