Okay, but this prediction will be impossible to judge. The reason being that Nintendo is also releasing New Super Mario Bros. 2, which is from a series that is almost guaranteed to boost HW sales. So we'll never be able to know if the XL in and of itself drove up sales.
However, when the DSLite+NSMB combo launched in 2006, nobody doubted the DS Lite's selling power. Then again the DS lite was a big design improvement over the DS phat. However, a similar argument can be made for the XL and its appeal.