I have the fs550 and have a few issues.
1. I cannot get it to go slow enough consistently even at the same sitting, it seems each time I start sewing it will either not move so i have to adjust it again and cannot do that in small increment so will be too fast.
2. Because of that, when I turn it off at the switch to make final stitch or two manually then press the treadle to turn by the hand, the motor still has energy in it so does an extra partial cycle of the needle. Which could make an unneeded hole.
3. Was told it would have plenty of torque but doesnt.
next time will look at another