I was proposing to solve the problem originally stated. I thought that my description of how to solve it using a heap was fairly clear on that point by telling you when to start removing elements while still adding them.
I agree that until N is large (causing, eg, memory problems) or the comparison function is expensive, using sort will be faster. The heap solution outlined is helpful only for large N. For an expensive comparison function, it is helpful once you start pulling off elements to keep track of the largest that you have in your set. Anything which is larger than or equal to that one should not be added. This reduces the number of comparisons significantly.
As for the sort utility, I had thought that it had an option to only return the largest N elements. But looking at it more carefully I don't see it. It must have been somewhere else that I saw that. My bad. :-(