Hi SM,
I did the MR on Sept. 20th, 2012, so sunrise/sunset should be ~same. I suggest leaving the trail head between 4:00 and 4:15. It takes approx. one hour to reach the ledges, and you begin to get light ~30 minutes prior to sunrise. I believe you will have sufficient light by the time you get to the ledges to not require your headlamp, although you may prefer to leave it on.
My experience was that all the pictures and videos I reviewed did not really prepare me for the hike. It just does not look the same. You can see my trip report for more detials: http://www.whitneyzone.com/wz/ubbthreads.php/topics/27939/First_Time_Mountaineer_s_Route#Post27939

I did hike with another person, and we each took different routes down. I went down the main trail, Carlos went back down the MR. We both arrived at the trail head at the same time. Carlos took his time, and took many picutres, so I "guess" the MR might be a bit faster, but it only took me 5.5 hours to get down the main trail, and eliminated downclimbing the final 400.
I really enjoyed the MR. I'm sure you will too.
Enjoy,
John