I doubt the yellow (blue tees being the traditional members' tees at NB) tees would have been shortened to anywhere near 5,800-odd yards during the summer, or at any time during the year for that matter. The club's website lists the yardage from the blue tees as 6,140 yards, with the whites being 6,500, and other than the new tees they've built in recent years (such as on the 6th and 12th) there isn't usually much difference between the members' tees and the competition tees so I suspect that everyday play is somewhere in the 6,200-6,300 yard range.
Mark - I wouldn't say that many of the greens are flat, there are a few that have a great deal of contour such as the 3rd, 7th and 12th; the 16th is unique and the rest all have subtle breaks and tiering. I take your point however in that 6, 8, 13 and 18 are, for the most part, relatively flat. I think the stretch from 5-8 (7 apart) is the less interesting part of the course rather than the front 9 being mundane as there is plenty going on elsewhere with 2, 3, 4, 7 and 9 all being pretty good holes in their own right.
Gullane #1 would be the obvious competition for NB for the second best course in East Lothian title however having played Gullane a few times recently I consider NB to be the better of the two by a distance.