Summer Tires?? 225/55/17??

The only issue I could see is it's a smaller tire circumference so your speedometer will read faster then you are actually going. You'll also turn slightly more engine rpms for a given speed which could lower your fuel economy on long road trips. It's not an extremely drastic change though. From the Miata.net tire size calculator:

Your tire diameter changes from 27.2" to 26.3" which comes to about a 3.1% difference. So if your speedometer reads 60mph, you'll actually only be going 58.1mph.

You will be putting more miles on the odometer then actually traveled so you'll need to compensate for mileage calculations accordingly. Because you are indicating more miles then actually traveled, you may think your fuel mileage improved drastically if you don't do the correction.

For example, for every 600 miles you put on the odometer, you've really only traveled 581 miles. Let's pretend you used 22 gallons of fuel (two 11 gallon fill-ups) to travel that "indicated" 600 miles:
600 / 22 = 27.2 mpg

However the corrected mileage is 581 (600 X 96.9% or .969):
581 / 22 = 26.4 mpg