Depending on the linkage setup you should be fine power wise. How long are the servo arms and control horns? With the 5645 output I would have no worse then 1:1.5 ratio between the two. Personally I found that the 5645 servos did not center all that well. The 5955 were much better and have now been replaced with the 7955. I think you are OK on power but after flying a few times if you find yourself chasing trims then you may want to upgrade. No need to do all at once, you could do elevators first and then ailerons later.