RE: evo xtv 70
I'm a bit unsure about what you want to say in your message. Is it correct that you have bought an EVO 70 xtv and now want advice about this board, especially about using biggers sails on it (5.7-5.8)?
I would say that if you are light, say 65kg or something, you can get away with using sails as big as that. You need a bigger fins though and it will not be a perfect ride, expecially not when you are not planing.
I have not tested the EVO 70 xtv for max practical sail size yet, but I used to ride the old EVO 70 with a 5.7 and I'm 69 kilos.
In any case, if would be interestng to know your weight.