Biggest problem you're going to run into is hub size. If the hub isn't the same, then you're hosed... cause the discs won't fit. If the hub is the same, then you're looking at getting a bracket made to mount the larger calipers. IIRC many of the components from the earlier 626 are shared across the line. Speed3/6 brakes are Volvo derived, prior to Fords sale of their stake in Mazda. IMO, you're probably best off finding a big brake kit from someone like SSBC or Baer, that features bigger rotors and a bracket to move the caliper accordingly.
You also cannot forget the master cylinder, or the brake balance. You start making parts bigger, you start messing with the balance, and then shit like a tail happy msp starts happening. Personally, i'd start with a set of uprated pads ( I really like the Hawks I put on the Mini), and go from there.