Chris last year while trying out my sails ,although I raised the boom on my main sail to clear of my rails and doing damage else where, I was at a loss as to a shortage of performance on the general speed of the boat till I noticed the low angel of the boom . I raised the boom to horizontal  and away she went like a good one . My boom is set about 6 inches above my head so the down ward angle to clear the rails was quite a lot , whether the raising of the boom let air underneath creating a vortex , or slack in the sail  gave an extra 2.5 knts I do not know , but my boom is horizontal or above when sailing now . cheers .

OK, thanks guys, both good points - no aerodynamic or other reasons relating to the way the sail sets, though?

Yes, helps to keep the sail out of the water when rolling or in a seaway.