I bought one of the early servomotors from Allbrands. At that time, they were only selling 1/4 hp motors, so that's what I got. I use it on my 111W155 and several other industrial sewing heads and have never had a problem with it being underpowered. When you consider that a home machine is somewhere in the 1/20 to 1/10 hp range, 1/4 hp is quite a step up and 3/4 hp is akin to dropping a Cadillac engine into an Escort.
One problem I have with my servomotor is that the speed changes are not smooth - there are about 5 "steps" from 0 to full speed and sometimes I want to sew in between two of the steps. I have never heard anyone else mention this, so perhaps it has been fixed in the later or more expensive models.