I don't really understand the desire to know the dead band is. It's narrow.
The dead band was really narrow on the 378 and they have all accuracy of a drunken sailor wearing a blindfold.
The futaba s3150s doesn't have a particularly narrow dead band but it is a pretty accurate servo and has a good quality reputation.
Whether a servo can rotate to 60 degrees in 0.10 or 0.11 seconds isn't too important. I admire those people who can tell the difference though.
It's well known that manufacturer figures are not very accurate anyway. Especially torque figures.
By all accounts so far, it seems like the 398 is up to task
Maybe it's the servo of the future?
The gear slop on the 378 wasn't anything to write home about. In that sense MKS are far ahead of the pack. Also is accuracy.