The time honored standard is 1.34 times the square root of the static waterline as a starting point for discussing hull speed. That doesn''t change with the point of sail. But the length of the waterline can change (lengthen) as a boat heels over, so yes, the boat can go faster than the standard 1.34 times it''s static waterline because guess what...........the waterline suddenly got longer!
And yes, some boats can do better than 1.34, and some can''t. My little 27 footer has exceeded it''s theoretical hullspeed for extended periods on several occasions, primarily downwind or reaching. The most recent time it turned out to be 1.7 times the static LWL.
Bottom line -- use 1.34 as the starting point, knowing up front that it may be high or low for a particular boat. Let''s don''t make this more difficult than it has to be.