Manufacturer's quoted range won't work for all sailor's weights and in all conditions. I believe most manufacturers stretch this range a bit (I mean, who would choose a 75cm wide iSonic122 to sail with a 6.0 ?)
A 60 kg sailor could get away with a max (5.5 or even larger) sail on a particular board, whereas you couldn't @ 102 kgs. But then again, when it's howling, there's no way a lightweight can hold things together with a 3.7 (on the same board) the way you could.
Btw, you are asking an awfull lot of one board if you want to use it with a 3.7 and a 5.4 (luckilly 3.7 days are really rare as I understand).