Well, first of all, it's going to be very hard to get one of those cards within the first couple weeks of release. They should sell out very quickly and get snapped up when new stock becomes available. So you may not have a choice but to wait and by that time, custom models may be available.
Even without that situation, you would still want to wait. With Pascal, custom cooling and enhanced power availability will make a big difference. There's also the possibility that custom cards will be cheaper than the Founder's Edition, which has an extra $100 tacked on to the base price. So by waiting, you should get more performance and more value.