Well, I tried this out on Sunday - seems to work well.
The 15-13 "gap" isn't noticeable - by the time you're going downhill enough to be that far down the cassette a 2 tooth step is fine. I haven't noticed a difference heading back up the cassette either.
Wanted: Trek XO2 54cm
Running:- Orange AluO; Orange Gringo; Orange Evo2; Trek Madone 6.9 SSL; Trek 1500 SLR
In Progress:- Orange Elite; Trek Elite 9.8
Selling:- Orange Sub3; Orange O2
Sold :- Merlin Titanium; Specialized 99 FSR Elite; Orange Mr XC; MountainGoat WhiskeyTownRacer; Orange AluO