Hi!
I'm going to order an "A+Case Monolize Midi Tower" case that sports 2 (!) 250mm case fans for my upcoming air-cooled overclocking PC.
Even if the standard 250mm fans included in the case should be really good, they'll need to be replaced at some point in time.
I agree, that there is only a very small market for 250mm fans at the moment. But it seems as if more and more cases include a 250mm fan.
My personal experience with moving from 80mm to 90mm and then to 120mm tells me, that 250mm fans should be able to move quite some amount of air at a very low noise level.