I can't give you specifics for the Zipps but I have some trainingwheels that are 23mm wide (outside) and a 23mm Conti 4ks is already 25-26mm on that one, so I can't imagine a 25 would be advisable from an aerodynamic standpoint. For comfort it will probably be nicer though, although the 23 is plenty comfortable for me at 8bar. I haven't mounted a Conti 25mm but have put on a Michelin Pro 4 Endurance 25mm and that was more like 28mm. Vittoria 23s I've found are more true to their label, or even on the narrow side, measuring 22-23 on a 23mm rim. Just some info to help you out, I'm sure others will have the Zipps mounted with Conti's, that has to be the most common combo.

Edit to add: Flo's wheels were windtunnel tested with Conti 23s as being most aero, just wanted to add that, another wide rim parameter for you.