WindGuru is great. It is pretty accurate as far as the ups and downs throughout the week but the numbers are always under the actual wind speeds. You normally have to add 5 to 10 especially if the wind direction is out of the east. I tell people to bring your full quiver to make sure you are covered. Basically everyone rides a 7 or 8 everyday but sometimes you would like to have that 5 or sometimes a 12 or 14.
Try this site: http://weather.mhpcc.edu/wrf/maui2/
This is probably the most accurate site I have found. It gives you a map of Maui County showing wind speeds and direction on an hour by hour basis for the next 2 days.