Yes. I found this paper which measured the errors of the system: https://apps.dtic.mil/sti/pdfs/ADA138283.pdf
They performed two kind of measurements. On a bench they measured the drift of the helium-flow gyroscope in itself, and then they took the whole system to drive test loops and measured the errors in eastings and northings.
Quite a thorough job in fact, but I guess that is what you get if you ask a rocket scientist to evaluate a new-fangled vehicle navigation tool.