A correct estimation of damage increase in WM would have to take into account the following:

The loss of auto attacks (100pot/3s) in minuet. Auto attacks have been roughly 25% of our damage pre-HW.
The difference in determination scaling of auto attacks and weaponskills (this was changed in HW)
The amount of skillspeed you have (the more Sks you have, the better WM should become compared to normal)
The estimated PPS difference of having access to Empyrean Arrow - its roughly +100 potency (compared to a heavy shot) every 15s
The difference of having access to Iron jaw (roughly 1 gcd saved every 18s)
The lost bloodletters in WM from only being able to use one oGCD inbetween skills

In short, you can't look at '20% damage' and take it at face value - in reality the bonus is FAR SMALLER than that, even in a dummy. My parses have shown around a 5% increase in dummy dps. Losing auto attacks is HUGE.