Yes it might be possible. But in the end, it's not easy (probably impossible) to get teh same range out of one boards as you get out of the 70+85l combo. So expect to give up a bit of comfort at both ends.
That said, the Q76 do handle a 5.7 sail as Ray writes. It becomes a bit more difficult to get planing early, ie you must in the usual way take more care not to load up the fins before you're up to speed. But it works. A set of 16cm front fins make it even better (they are a bit stiffer not only bigger). Underpowered 5.3 is no worries at all though. Perfect Sweets Spot for this board.
Here's the Q76 doing its thing in underpowered 5.5:
4.2 is OK too. It gets hectic when you ride the wave, especially if you're at the same time overpowered. But 4.2 winds is still not enough to blow the Q76 out of the water, I reckon. I'm your weight almost exactly btw.