Ryanair at the mo have a limitation for the 800 of 6500ft for planning purposes.
Eg the 800 does not do DUB Aberdeen because its only 6000ft yet have seen BA, BY767-300s, Sultan of Brunie A340 in there.
Its a company thing not a Boeing thing.
When was in Seattle think I can recollect a Boeing engineer stating that every Boeing built is designed to stop in 3000ft if it had to. Dont quote me it was a few years ago!