As far I noticed it deppends on the track, but not just the MX5. For example the GT40 is a whole 3-4 seconds slower on Road America and 1-1,5 sec faster on Laguna Seca.
They changed the tyre model for the MX5 and for a couple of other cars to the new version. But I was faster with the new one then the old, at least on Lime Rock.
There isn't any roadcars in iRacing, all of them are trackday or racing cars. With the arrival of the McLaren MP4-12C GT3 you can compare both of them, maybe I renew my iRacing account for this too, since I got a code for it - 3 month for 1 price for everyone.
Well I trying it on Silverstone, but I can do only a .5 sec faster laps with semi-slicks, and I can't really feel the difference in grip, nor in braking, nor at apex, nor the exit.