I've read the darned Little Green Book several times and haven't seen a thing addressing mag brakes one way or another. So I assume its okay to make that modification.
(I do know that at least one guy who left a message here on the forum who received a factory stock gun, an 85 Combat, that did NOT have a mag brake installed. When he talked with Mike, the gunsmith at CZ, he was told that some where shipped that way, and that it was no big deal.)
I have a pre-B 75 that I'm shooting a little now -- it still needs some work. I removed the mag brake from it, and it works fine -- but I think it could be a problem in a match -- as unless you insert the mag just right, you may find that it could cost you more time than you save.
That said, you can tweak the exiting mag brake by shortening the top coil (pull the pin, use needlenose pliers, tighten the loop or bend it the other way). The metal is brittle and might break, but several have done this with good success. You can also order the mag brake for the 85-Combat from CZ and install that. The cost of shipping will be more than the part.