MX5 Brake Upgrade
The standard 9 inch discs look rather weedy in a 15 inch alloy wheel & the standard calipers are not exactly pretty. Also with a 200bhp+ engine you are asking for brake fade if you're ever on a mission or on track. 
Answer is to get some bigger discs and 4 pot alloy calipers in.

Brake fade?
Brakes convert the car's kinetic energy into heat. They are engineered so the discs get hot rather than the pads, and the discs have radial holes (the front discs are "vented") so that air is pumped through them as they rotate to help dissipate this heat. A larger engine is capable of supplying more kinetic energy for the brakes to soak up. If the discs are too small, they get too hot. When they're too hot the pads are less effective so increased pedal pressure is required to slow the car. Quite simply bigger brake discs are a bigger heatsink and will dissipate more energy than a small disc before they get too hot. They have a larger surface area, they pump air through the vents (which are larger anyway) more effieciently so they blow off heat much better. The APracing discs are directional with swept back vents for improved air pumping.
High brake temperatures can have much worse consequences than fade - they can cause the water in old brake fluid to boil and this means complete brake failure - pedal goes to floor with no resistance.

Note the caliper has no bearing on brake fade at all. If you put a bigger caliper on the same old brake disc, it will probably fade worse because the caliper and pads are reducing the brake area available for cooling.

What's available?
There are kits from FM and Brainstorm in the states, these are quite expensive and the FM kit doesn't address the resultant balance problem when you have the same old brakes on the back.
Both these kits use Wilwood calipers. These are somewhat cheaper than the APracing calipers I went for.

My system
I used APracing 5100 series 4 pot calipers. A bracket had to be made to mate these with the bolt holes on the front suspension upright. The original brake hose and banjo bolt from the old MX5 calipers can be re- used in the new calipers, you have to turn the banjo round so the locating pin doesn't foul on the caliper body.
I originally used nissan 100nx discs re-drilled for the MX5 wheel stud pattern. These discs were extremely heavy (6.6kg) and had an over- large swept area and were designed for a larger hub flange, and they squealed loudly sometimes, so they were put on temporarily while APracing discs were sourced and alloy mounting bells designed and manufactured. The discs are 280mm diameter and 22mm thick. With the alloy bells the APracing discs are 1.8kg lighter than the nissan ones. per corner.

Front/ rear bias
Several factors affect this. And there is the back brake proportioning valve to contend with. The stock brakes generally put a little too much bias to the front, this is to stop us all doing "handbrake turns" unexpectedly. larger front brakes will have a larger brake force (larger piston area) operating at a larger radius (bigger discs) resulting in considerably more front bias again.

standard front caliper piston diameter 51.1mm

effective standard disc radius 90mm (radius to centre of piston)
new caliper piston diameter 38.1mm x2
effective new disc diameter 115mm 
So braking torque increased by 2x38.1²/51.1²x115/90 = x1.42

I installed the larger 2001 rear discs and caliper brackets taking the rear brake discs up to 11 inches (from 9 inches) to maintain a reasonable front/rear bias. Note, much larger pads are included with this upgrade (though they still fit in the standard caliper)

rear caliper piston diameter 31.75mm
effective standard disc radius 100mm
effective new disc diameter 125mm
braking torque increased by x1.25

note front rear bias with no proportioning valve would be 51.1²/31.75²x90/100 = x2.33 i.e. 70/30
with new front and rear brakes bias is x2.65 i.e 72.6/27.4

Why is there a proportioning valve?
The centre of gravity of an MX5 is about 16inches up. This means that under braking the 52/48 weight distribution ends up with more weight on the front wheels and less on the back. The wheelbase is 89inches. A braking acceleration of 1G (dry tarmac!) alters the weight distribution to 70/30 fr/re. 

If the brake bias was set to permanently put over twice as much torque to the front wheels as the back then consider the situation where you are braking on ice. lets say the coeficient of friction is 0.1. Best possible deceleration would be 0.1G in which case the weight distribution is 54/46 fr/re. Now as soon as you touch the brakes, the fronts lock, you've lost steering and the rears aren't slowing you down at all.

So, at low pedal pressures you want about the same braking force front as rear. At high braking forces you want twice as much on the front as the rear. The proportioning valve tries to achieve this, at low fluid pressure the pressure to the back brakes is the same as the front (up to about 400psi) but when the direct fluid pressure to the front brakes has gone up to 1000psi, the pressure to the rears has been limited by the proportioning valve to 600psi (approximate figures taken from test spec in mazda MX5 service manual).
I now have generally increased braking resulting from lower pedal pressure (x1.42 front, x1.25 rear) so the increase in front bias is mitigated on tarmac by the reduced effect of the proportioning valve resulting. On ice & snow I'm simply slightly more likely to lock the front wheels.

Parts from APracing
calipers     CP5100-802S4
                CP5100-803S4 Note these are handed and directional & specd for trailing mount
pads         MDB1864M1155 Mintex M1155 pads
discs         CP4530-752G4
                CP4530-753G4 280x22 discs, 8 bolt pattern 4 grooves. discs are handed too

Parts to be fabricated
caliper brackets
mild steel zinc plate/gold passivate
disc bells
Aluminium alloy HE30 Hard anodised

The nissan discs needed extra wheel stud holes drilling on the mazda 100mm PCD

This photo compares the stock brake disc with the 11inch one.

The other snap shows a trial fitting of the system with the nissan discs

Finally, the alloy disc bells have been completed, the bare disc is shown below & a view "behind a wheel". Braking is sharper than stock, due to larger piston area and radius, as expected & absolutely no fade, though to be honest I never noticed any with stock brakes either....