Nothing wrong w/ 5800 & Open Sport in a proper hand built wheelset but you would be getting 6800 hubs vs. 5800 as well as the Open Pro rims.
I take the long view, having the better hubs for a re-build in future is a good investment in my book if it doesn't bust the budget.
Either way Shimano hubs and Mavic rims are well proven quality gear, not the fad du jour.

-Bandera