You want to keep the same OD.
There are a number of web sites that make this calculation easy.
Here is one:  http://www.1010tires.com/TireSizeCalculator.asp
I get:
Overall Diameter: 25.32 in 643.12 mm
Overall Diameter: 25.47 in 646.93 mm
It's close, your ODO/Speedo would be about 1% lower/slower according to
my calcs.
Getting the lower profile tires may be an expensive problem.

You must like a rough ride!

You can use this tire calculator to see effects of tire size changes on
speedometer readings: http://www.miata.net/garage/tirecalc.html

Pluggin the two tires into the calculator, they are within 0.6% of each
other in speedometer/odometer readings (tread OD is all that matters for
that).  So when your speedometer shows 60 mph, you would be going 60.4
mph - acceptable error.  Fuel mileage will also be changed by that
amount (insignificant increase of 0.6% - which may be offset by
increased weight of the larger wheel - see next paragraph).

Larger wheels will usually be heavier, and the weight will be
distributed more to the outside of its diameter (IOW - rotational moment
of inertia - resistance to increasing and decreasing wheel rpm - will
increase), so you will have a penalty in acceleration and braking.  And
your ride will be harsher (shorter/stiffer tire sidewall).  So you will
be giving up performance and ride for what you consider better looks and
cornering ability.

Oh - and tire prices go up exponentially with wheel size.

Your only gain will be in appearance (and bleeding edge cornering
ability).  Everything else (tire cost, ride quality, acceleration,
braking) will suffer.  Is that what you want?

Bill Putney
(To reply by e-mail, replace the last letter of the alphabet in my
address with the letter 'x')

Everybody follow Bill's advice.  Changing anything on the chassis can lead
to problems, engineering-wise, legal-wise and a violation of school rules.

As for appearance, its a cultural thing.  My kid's school says no red
shirts because its a gang color for the Bloods.  They say no blue shirts
because the Crips use that one.  I told him red and blue are just points on
the electromagnetic spectrum, that's all, for us white folk.  I hear the
gangs go for those 19 - 22 inch wheels with the skinny rubber, so if you
plan to hand the car down to your kid, the school may not allow him to park
it on the grounds.