PonchoBuick -
I went here :
(Miata)
Tire size calculator
And made this comparison :
I also went here :
Car Bibles : The Wheel and Tyre Bible Page 2 of 2
And got the same result :
From the above, the revolutions per mile look very close. Still, I think you should do a GPS comparison to your odometer. That's what I have done :
My Continental Contact-Pro Tires
I am not sure how to quantify the width, but I agree that performance tires have a bigger contact patch and are more sticky (increased Rolling Resistance). Because the tire diameters are so close, I think you made a good choice.
CarloSW2