Speaking form experience and by the number of broken HiTec servos I have (lets just say lots), the HS-5646WP will not cut it. I have broken my fair share of Thrusters as well and that feedback has been given to Rusty which lead to the stronger thrusters we have now.
I now run Futaba BLS172SV and never had one fail. I run 2 T200’s on steerable mounts and the torque generated when not only at full thrust but also turning a heavy boat (mine is over 25kg fully loaded) requires much stronger servos that the Hitec WP ones. I also have slowed down the servo Speed to ease the stress.
I use a Castle Creations BEC and don’t use the built in BEC off the Afro ESC as I found out early that when the thrusters were near full speed, the built in BEC couldn’t handle the current of a pair of Hitec WP units.
I have also used the Savox units (SV-1270) wich have been proven to be reliable. I know the high torque servos are more expensive but they are more reliable long term.