If geared right the 550's will run well, however I feel on the stock Swift16 530's are the way to go.
Century are shortly bringing out dedicated 550 and 620 models, which will run that specific blade lengths and have booms to suit. While the Swift16 will accomodate 550's, say you wanted to go up in tail blade length you may then get problems.
When it comes to bogging this can be avoided by pitching less, as the Swift will ultimately stay the same weight, but present a lighter disk loading using the 550's you could run a degree or two less of pitch and not have problems.
Also the motor you run will dictate what gearing is required to avoid any power lose during hard, pitch pumping manouvers.
Agility wise, with hard dampers and the metal head block it will rock and roll with 530 or 550's. The last mod I fitted to mine is the metal head block and you can feel the extra precision during 3D, a quick stab of cyclic to roll the model on it's side and it does so and locks in knife edge.
Best answer would be to try the 550's if you could borrow a pair perhaps, gear a tooth or two lower than required for the 515/530's and start from there. From there decide what you like yourself on the model.
Team Robbe SchluterUK