CutCutCut: I hadn't heard of grandstream before, I'll check them out. Any particular models you've tried?
Grandstream GWN7000* and GWN7610* are a good combo.
I can vouch for that. Only have a small house (~123m²) and it has two storeys. It sits on a 952m² (quarter acre) section. I have a centrally located GWN7610 which provides good coverage throughout the house and right around the section. My house (at least the downstairs section) was built ~1853 so the walls are very thick and solid and I have absolutely no problems with wifi coverage anywhere in the house.
* Took the liberty of correcting the model numbers.
The other thing to consider is that 60GHz may start to make the cat6 not able to cope, so could be fiber based or need newer cabling on future AP's. Even current dual 5GHz radio stuff is already over what gigabit can do, and 10g copper is really fussy about cabling etc.