Matt:
I'm not sure the data bears you out re. your arguments on 15 and 13.
-- 15 this year played as the easiest hole on the course, at 4.7762 strokes. That's in line with its historic average, both in terms of how easy it played relative to the other 17 holes as well as its average score. No. 15 has historically played as the easiest hole, with an average score of 4.8. See here (course records from 1942-2007):
http://www.masters.org/en_US/history/records/alltimestats.html-- 13 played a bit tougher, but not by leaps and bounds; the difference is a degree or so. It played as the 15th hardest hole this year with a stroke average of 4.8375; historically it's played as the 17th hardest hole with an average of 4.8. In essence, it was only marginally more difficult this year, and moved up the list because the front-nine par 5s played slightly easier than 13 this year.
-- What's interesting to me is that, in the aggregate, the course's par 4s have become harder relative to the other holes on the course, based on historical averages. Historically, four of the courses par 4s (3, 7, 9 and 17) have been among the nine easiest holes on the course, with the remaining six par 4s (10, 11, 5, 18, 1, and 14) playing among the nine hardest holes. This year, only holes 3 and 14 played as "easy" holes (easiest nine holes on the course). Eight par 4s ranked among the toughest holes to play.
-- Notably, holes 1, 7, 9 and 11 have become more difficult than their historic averages. For instance, no. 7 historically ranks as only the 12th most difficult hole on the course; it ranked 3rd this year, and previous threads have suggested this hole has perhaps been altered more than any other at Augusta. Only one par 4 -- 14 -- played significantly easier this year than its historical average (it used to be the 6th hardest of the course's 10 par 4s; it was the second easiest par 4 this year).
-- Truly interesting, to me, is the evolution of no. 12, maybe the course's most famous hole. It historically averages 3.3 strokes; this year, it played at 3.0939 strokes (in a year when scoring averages were a half-stroke easier than historical averages -- 73.79 this year compared to 74.24 historically). No. 12 historically has ranked as the 2nd most difficult hole on the course; this year, it played as the 13th most difficult hole. Has technology finally caught up to the famed 12th? I think most players were hitting 9-irons into 12 this year; did it used to be 7-irons, and has that made what was once a feared hole into one that is -- for the Masters -- a relatively benign one?
On Matt's point, I'd argue the lengthening and changes to 13 and 15 have kept those holes as premier risk/reward holes, when keeping them at their pre-2002 lengths/features would lead to everyone going at them in two. I enjoyed watching how folks attacked them in different ways.
To me, the main alterations on the course that have affected scoring have come on the par 4s, which in the main have become much more difficult, and made it seemingly more difficult for players to go on a long under-par run of holes.