Given a constant speed and assuming you use the complete width of the deck for each cut, the percent change in deck width is directly related to the time it takes to cut the given area.
So, if it takes me 10 hours to cut everything with a 38" deck, and a 54" deck is 42% larger, it will take me 7.04 hours to cut it with the larger deck.
These numbers would change if you assume you always have a 5" overlap with each cut. In this situation, with the larger deck the 5" overlap becomes smaller (relative to the deck width), so you are "wasting" less. The 38" deck becomes a 33" deck and the 54" deck becomes a 49" deck. So, 10 hours becomes 6.76 hours.
I could keep making this more and more complicated if you would like, but with out real data (overlap, speed, how much trees hurt me), I don't really know.
Austin