Do not drill the valve hole larger in the Zipp as it will void the warranty. The best thing to do in my opinion is that when you are mounting the tire stretch the areas right next to the valve while mounting. You can probably also cut away some of the extra material if you were careful, but I personally wouldn't do it because I found a technique that already works for me.
Apologies for the thread hijack but thanks for the response. I know some people widen the valve hole with a countersink and deburring tool but like you suggest, I don't really want to go there. Guess I'll try to remove a bit of base tape with something. I'll also try the stretching you mention, but this tire in particular seems to have an extra thick overlap of base tape around the valve which doesn't sit well in the 303. IME anyways. Going to be mounting one this week so any other thoughts are welcomed!
On thread topic, I do agree the SC grip better than CX but do not last as long, so there may be something to the "more fragile" comment, however, I have used them in a few rough roubaix type races and have never flatted even with a worn rear tire.