I don't see anywhere in your post where you state the beam angle of the SourceFour™. I also cannot believe the S4 had a 750W lamp. I used the Berkey Colortran ZoomMini extensively in the mid-late 1980s, and preferred it to the "full-size" BC Zoom Ellipse. This was with an AV company who felt that since Navitar 6"-9" with or without Barlows worked for E-IIIs, they should buy zooms for ERSs also. I was hired shortly after they purchased the Colortran gear, or things may have been different.
You're speaking of the Colortran ZoomMini 25/50, so let's see what the manufacturers' spec. sheets say, in comparison to an HPL750/115v ETC 426, 436, and 450. In this thread,
Colortran Photometrics, I posted information from the original Berkey Colortran cut sheets.
Below is the Excel PDF. Every S4 should have kicked the little guy's butt; that doesn't mean the ZoomMini is a bad fixture. Quite a different beast than the Mini Ellipse 20/30/40, with screwbase 500W lamp.