I was there in August/September last year and it blew nearly every day. Unfortunately I didn't take my kit because most of it was broken, and I was travelling to loads of different places and not just Fiji. I seriously regret not taking it now.
The Yasawas are awesome but I think there's only a few spots that are really suited to kiting because of the prevailing wind and launching/landing areas. Try Sunset Waya - I know this is kiteable. You should go there anyway because the guy who runs it - Moses - is an absolute legend! He is so funny and has some great party tricks. Check out the pics;
I'm not too sure about Mana island. There is a big lagoon which is super-flat water but as I remember the lagoon is partially sheltered from the prevailing wind.
Good luck and if u want more info just ask
Cheers
