Don't know if anyone here has done this yet but ill share my expereience. Last weekend i finally decided to fit the cb650 cam iv had lying round into my 550. Has gave it a nice little bump in power, not loads quicker but it seems to have more power in places it didn't before due to the higher lift and longer duration, so it feels well more usable.
The job was pretty straight forward, no different to refitting a 550 cam, times up exactly the same, all i did have to do was dremel a tiny bit of casting off the inside of the head for a lobe to clear.
Would recommend this swap to anyone, cost me 40 quid, alot less than an aftermarket cam.