Hi,
you're right steppers lose torque at speed, low inductance units like these lose less than most but still only have a 1/4 of their torque at 1000 rpm, so
about 2Nm or 305 oz.in.
I've found a spec for the largest of the integrated servos from JMC, 180W and 0.57 Nm or 87 oz.in. The servos can temporaily do three to four times their
continuous rated torque but they'll fault out in 20-30 seconds. If you expect to run them in excess of their continuous rating you'll be disappointed.
Quite frankly I doubt 87 oz.in is enough for a substantial machine. If they lag behind because they don't have the torque to maintain the position/velocity
profile of your Gcode trajectory they will try to keep up and either overheat and fault 'over temp' or lag behind the position required and fault 'following error'
Closed loop feedback does not make servos immune from being overloaded and when overloaded they perform just as badly as any other overloaded motor.
and when its right konfigured the have also much power at low feed rates
wrong...they have 0.57Nm torque from 0 to 3000 rpm, they do not
have more torque when going slow.
You might be advised to get one and try it out before you buy the whole lot.
Craig