It does have a 4 inch pulley on it. Only came with one size.
I know all to well what you are saying is true about practice. I have done 4 interiors in the years I had the Thompson Mini Walker. It ran wide open at 900 spm. The reason I bought it to start with was I had a dear friend who was a trimmer and I knew how difficult it was for me control the clutch. It really was easy to control while doing tight work. Unfortunately, as I got more experienced, I found that it just did not have the presser foot height to handle pleats with sew foam panels. Also, since it was not a needle feed, as the material got thicker the stitches got shorter.
Do take a look at the picture again though. If I move the rod toward the pivot point of the motor it is going to get more sensitive. I could move the rod toward the pivot point of the treadle and desensitize it but I really don't think that is the issue. This motor just does not have much variation in its speed between stop and what ever setting you put it on........Wish we were neighbors.