there are two new details:
1) While waterstarting, I noticed that the sail had much power to pull me up on the board fastly, so I thought that it was not just matter of sail.
2) I asked the same question to "a leading sails manufacturer" and just got the following reply:
I think it doesn't have much to do with the sail not having enough power. When using a board that is a little too big for the sail then the board will not want to go upwind as well as it will seem that the power is not getting to the fin, making the power in the sail not being transferred and for this reason you are not feeling it.
I think that even the 100 Liter board might a little too big for this sail, specially as the core has a little short of a boom and the power in the frond of the sail. If you where to go for an Alpha then the center of effort would be a little further back meaning more power would be transferred to the fin and you would be able to get upwind on a little bigger board then you could on the Core."
As far as it seems, I should FIRST buy a smaller board (100 lt.) an THEN upgrade my 4.7 to a power wave sail (as the Alpha).
Do you agree ?
Thank you, regards