Chris, I think it is the boom too high (too much lift of the board from the sail?). All the other things you suggest I am already aware of, so it must be that one.
With that set up I have very good speed because the board is out of the water, but probably the tradeoff is the upwind ability...it's not easy to find the right compromise!
Anyway, that's just theory, I haven't had the chance to try in the water.
Mim, I totally agree with what you say (stand up, flat sail and open) but that's overpowered mode, in those conditions it's easier to gain upwind.
In slightly underpowered to well powered mode a more fat profile to the sail _should_ increase the upwind angle, but it also loads more the back foot... does it help (by experience)?