I am a colleague of Arthur
Weight is not exactly the right word and not an issue; cost & complexity of the Kollsman unit are potentially an issue.
In order to use the GPS method for establishing Position Error we need to fly at a constant airspeed to keep the calculus simple. Unfortunately the scale and pointer (too wide) of the original aircraft instruments do not allow this. Any deviation in the Indicated Air Speed flown will immediately translate into a pressure error. The 3-digit counter of the Kollsman unit would show any deviation from "target" IAS. The accuracy of the ASI is less important as the standard cockpit fit is not that accurate; hysteresis would be a problem.
As this is part of a simple flight test for STC certification we want to keep it as simple as possible. A pressure transducer is an option, but one we would like to avoid at this stage. Using a Garmin GPS would not get us any closer to establishing PEC.