After talking to Dan last night I think we're onto a few different tracks.
1. The (270mm) MX5 discs will maintain the same centre line as the stock discs. From a distribution of force point of view (engineering certification) this is important. It also means that we have a "easy" point of measure to establish clearances on suspension and wheels.
2. RX7 (s5 4 spot) Calipers are wider (obviously) with a lot more caliper on the outside. This may foul on some wheel combinations. The stock brakes are maybe 25mm higher than the outer surface of the disc.
3. Astinas/Lasers (BG, BA KH,KJ) use leading calipers - that is they mount to the front side. MX5 uses trailing calipers. This is no great drama, but may require the calipers to be swapped L<>R
4. The MX5 uses a different caliper design. They mount the calipers to an assembly that looks like this:
and
While the stock brakes are a one piece type caliper that is less efficient. The floating MX5 kind is better.
Interestingly according to most parts resellers all Mk2/2.5 1.8L MX5 calipers are the same. This means that the brackets and pads again are our only variance.
5. We cannot replicate the Caliper brackets from the MX, but what's the chance that Mazda kept the caliper mounting setup the same across the range (within reason)? This is the next point of measure. To measure the mounting bolt spacing on the Hub carriers and knuckles of a few different Mazdas. Their relationship to the disc centre line and radial positioning
6. From looking at pics of the Genuine Mazda pads between the two different sized MX5 setups the pads have a fair bit more surface area.
In short I think the swap is valid, is most likely going to deliver better stopping - especially when you consider the range of MX5 pads and rotors out there. A conversion to a larger MX5 configuration my be possible for more $$$
The 4 spot calipers are definately an option but now it all comes down to measuring. However what we're looking for now is a Mazda/ish caliper that clamps a 22mm thick disc of about 270mm that is either floating or one-piece. mmmm Bravo? E series? 929? Cosmo?
A.