I want to replace the twin servos on the SES arm shoulder with a single more powerful servo. Would the HSR-5980 be a good choice?
The 5980 is deffinately a good choice, however, I’d go with the 5990. For less than 20 bucks more, you get the built in heatsink, deffinately bang for your buck becuase it allows the servo to be run more than just a few minutes, compared to the 5980 
I have never tried to use a single 5990 in the shoulder, but we just built an SES Arm with 5990’s in the shoulder, elbow and wrist. It can lift 20 ounces easy. Here is a short video of the arm in action. 8)
WOOOOA! Thats impressive, took it like a champ! 8)
I wanna see its max capabilities. 417oz is close to robo-one jumping powers.
And i always thought the 5990 had black wires, weird?
Yow! That is indeed an impressive video. Gotta get me some of those 5990s right quick.
An arm lifting 20 ounces at 6" from the 5990’s pivot point IS pretty much at the maximum output for these servos. What more are you looking for? 
The 5990 does have black wires. There are other servos in the arm. 5475s in the gripper and wrist rotate.