OK, so I finally installed the wheels this weekend.
There is only about 2mm clearance between the brake caliper and wheel spoke. It's VERY tight. However, this seems to be the minimum recommended clearance and we've had no problems so far. I was going to buy 3mm spacers for the front but it seems I don't need them.
The centerbore is exact. It's very reassuring to watch them sung up tightly as the lugs are tightened. Also, the stock lugs have about 10.5 turns, far above the minimum 7 for safe engagement.
So I have to say, these are perfect winter wheels. Mazda 3s are basically everywhere, so these 17s should be around for a long time on the used market. A fantastic outcome.