Why make this UI mod? This mod would increase the efficiency of COMBAT SACU's via the veterancy mechanic through a reduction of user APM.
There are two types of Combat SACU's. 1. The type directly produced from the Gateway as a preset. 2. The type manually made via upgrades from a default (unupgraded) SACU.
The second type is usually superior for the following reasons: Immediate resource production, quicker build power available, less mass killed required to vet (this is the main reason for the mod). However the first type is generally easier on the APM (about 8 clicks less per unit).
Having a mod that automatically, or on keybind press, upgrades basic SACU's produced from a gateway into a "preset" configuration would combine the best of both types. Higher efficiency and lower APM.
To state how much more efficient it is to have a non-preset upgraded into a Combat preset, it's about 5 times more efficient. This is because the veterancy is based upon the units build cost when it's spawned in and doesn't factor in upgrade costs. Thus you need significantly less mass killed to vet up SACU's if you bothered to manually upgrade them.
To summarise, this UI mod would allow default SACU's to be easily upgraded into Combat SACU's and thereby benefit from requiring up to ~5 times less mass killed to increase veterancy.