OMathisen, I'm in the exact same boat you are in with the same size tires. The MT ET Street Radial II looks like they may be an option for the rear. The 275/50/16 differences are there but minor use this tire size calculator to show the difference
http://www.miata.net/garage/tirecalc.html .4" difference in hieght. which will throw your speedometer off by 1.2% when it says 60 you're going 60.7mph.
For the front the MT ET Street Radial comes in a 255/50/16 which also is .4" taller than the ones we currently use. I've been looking into this for a couple of months now since I'm a cheapskate and don't want to switch over to 17" rims. Hope this helps!
If anybody out there has done this swap please let us know the results!