Very quick calculations from http://www.miata.net/garage/tirecalc.html
+2.7% - Rays (26" vs 26.7")
+3.3% - My 18" Rotas (24.8" vs 25.6")
-0.5% - Your new ones (26.7" vs 26.6")
See the problem? The front to back stagger should be about 2.5-3.5% from what I've seen (somewhere around there) with the rear tyre being bigger than the front. With your -0.5% difference, your front tyre is bigger than the rear, the rear thinks its spinning quicker than it really is in relation to the front, and thus the TC kicks in as it thinks the tyres are sliiping - measurement of slip of course being that the rear wheel is spinning quicker than the front. You dont have to keep it the same RR as stock, as my example shows, my Rotas are actually smaller RR, but keep the size stagger, so TC doesnt get upset.
Hope whoever you got them from can appreciate they supplied the wrong tyres for a Zed - but perfectly acceptable for other cars with the same fitment. Hence why I asked if they were advertised as the right size tyres for the Zed, not the wheels.
Hope that helps