Some good points Danzer, thats what is great about this forum. To answer your points: Yes water could collect in the rear of the wing tanks and be dumped into the system in level flight, but if that happened I believe it would be caught and trapped in the bottom of the header tank (providing there is enough room because you emptied it on your preflight) and could then be drained off at the next preflight. Unless there is such a large amount of water due to a long storage period that all bets are off and you really should have thoroughly drained all tanks in a level attitude before flight. I think any large amount of water that could overwhelm the trapping capacity of the header tank would also do the same to a gascolator. I also have a suspicion that the header tank may even possibly do a better job of separating water because it is bigger and the fuel flow turbulence thru it is less than in the smaller gascolator.

We are all just brainstorming here but trying to use sound engineering logic. As you say, a thorough and consistent preflight is still the most important thing of all, taking into account how long the aircraft has sat and in what conditions-outdoor, hi humidity, etc.