I always read this thread with interest but have not posted lately because I did not feel I had anything more to contribute.
What I noticed most about changing to a 50mm pulley on my Adler 267 was the increased power at lower speeds. The smaller pulley does cut the speed in half but also doubles the mechanical advantage of the motor giving you much more control at lower speed. The machine will still run as fast as you will ever need.
Dan is the professional and his advise is always right on. I am just offering my observations as a guy who does his personal sewing and has made this change. It has really been a good thing for me.