Front rotor adaptor bracket

Post-mount disc brakes are all made to the same mounting standard. That's why you can whip a shimano brake off a Fox post-mount fork and put an avid on in its place without any modification. Provided the adapter is designed for a 203mm rotor (this should be stamped somewhere on it the adapter) it should be fine.

Aren't Avids a bit special, or is that difference in the caliper? They've got those Tri-Align washers that go between the caliper and the bracket.

The offset required for the spherical washers is in the Avid caliper, so the important dimensions of the mountings should be equivalent.

As I mentioned above, we can use the example of Fox, who only make one type of post mount to suit any company's brakes. To that you would fit either a Shimano caliper or an Avid caliper with the washers. A long as you use the washers with the Avid caliper then the two should be equivalent (for the same diameter rotor).

Now: Caveat Emptor: Different rotors have slightly different thicknesses and diameters even if they are made to the same "standard". This means that a certain amount of adjustment (with thin washers/shims etc) may be necessary to prevent the rotor from topping out on the caliper body. Everyone's mileage varies in this respect but it is only a matter of fine adjustment.

Also, though it won't be a problem here since you are going for a larger disc, if you were to go for a smaller disc there could be interference between the mounting and the caliper body. In that case it is better to try to match the caliper and mount.