The biggest difference between the Defender and the Officer's ACP is not the extra half-inch of barrel in the latter. The latter also has a bushing barrel with a thin flange to lock into the slide. The Officer's ACP has been known to have the bushing shear the flange, sending the entire recoil system downrange. The shorter slide uses a stronger spring and assembly of the Defender can be a PITA as the slide stop has to be installed while aligning the notch in the slide with the slide under spring pressure.
After trying more Officer's ACP and Defender pistols that I care to admit, my considered conclusion is nothing shorter than four-inches for me. That being said, I only have three 1911-pattern pistols in the safe with shorter than five-inch barrels. One is Commander-length with a bushing barrel and two are four-inch bull barrels.