Pete,
I would just swap out the 5645's for the 5955's and it will probably solve your problem. The 5645 is ok for smaller prop 3D and scale models or for something like the flaps where you need some grunt but are not as particular about the centering. I put 7955's in the tail of my Aviation Design F-16 and they seem to work fine. I have JR 8611's in my Boomerang and they work just as well but not any better (at least to me). On the plus side it is so much easier to set up the internal linkages for the elevators using the 7955's that can be programmed. I haven't been into jets all that long so can't comment on how long servos last for this applicaiton. But I can tell you that my 3 year old 35% Wild Hare Sukhoi is on it's original set of 5955's with no failures or gear slop and that I have already had to replace the gears in the JR 8611's in the tail of my 35% Comp Arf Extra due to gear wear after only one year. In the end run I think you are pretty much shooting yourself in the foot if you try to set up something as fast and expensive as a Jet with anything less than the best servo offered by JR, Hitec, Futaba or whoever you feel comfortable with. Just my 2 cents worth.