Passed my provincial safety check. I am sitting very low at the moment and want to go to 15" wheels, which is essentially what the MK is designed for anyways (MX5 running gear). Yokohama AD08 is a highly rated tyre for street and track; any good for us?

I thought you'd use snow tyres year round in Canadia.
What are your road surfaces like?

Let's put it this way: you have NOTHING to complain about regarding road conditions. I can't use my 13's on the surfaces we have due to lack of clearance. The winter kills the roadways.

Then choose your profile carefully.

Many 15" tyres give very similar overall radius as 13" wheels, especially if you have 50 profile or less.

Have a look here for comparisons
http://www.alloywheels.com/tyrecalc.asp

EDIT and if 13" wheels cause issues with the roads, you are probably going to need at least a 60 profile, maybe more!

[Edited on 3/11/11 by cliftyhanger]

Excellent link!

It will still help a bit on the clearance, and will likely fit under my front wings. This a a bit of a concern because I have no idea where to get new wing stays for the MX5 hubs, and the present ones are on Carbon Mods (very expensive) wings (which sadly, have cracked).

Actually 13" rims or possibly 14" ones with High ratio tires 80's likely (forget 60's :-) would be a V good choice for performance as well as ride and road clearance.
Big wheels with thin tires are for Posers... pure and simple.
Ever notice how F1 tire are high ratio and the rims are surprisingly small. Results of serious research and $$$.
Larger wheels have higher rotational mass (regardless of material used) and the mass reduction benefits of fitting smaller diameter wheels far offsets the wee bit of extra sidewall rubber of a larger tire .
Go Big = Go slow :-)

But on the other side of the hat it is very hard to get decent road tyres in 13inch!! so depends where you want to compromise! (well it is in NZ and other countries, so going to a bigger wheel gives you a lot more options for wheels and tyres)

Can't offer any personal experience on the AD08, but from what I have read its in the top range of road tyres without going to semi slick, and seems to have very good reviews in the autocross markets (also running 15inch). I am going to get a set of Bridgestone RE11 (as that is what is available here) and have heard from a couple of Aussie guys that the Toyo R1R are also very good (all of these tyres are in the top tier of the road tyre market)

I have RE11's on my Audi A4 Avant. Total overkill for the road only, expensive, but they are great and have lasted two full seasons.