Very surprised at your comments as SITA winds on the flight plan have a very high reputation among pilots who use these plans. I have been on the flight deck recently and the Captain was showing me that at the altitude we were at the reality and the plan was within 1 knot of each other. You seem a little vague as to who is your provider. Winds come from the met office and the source is from radio sondes. All the flight planning providers have good and not so good points. The overall accuracy of SITA winds on the flight plan is one of the good points of that provider. If you are certain of your facts get your dispatch department to investigate your findings as it doesn't sound right to me based on the facts you have given.