Quote from JEPPESEN - GENERAL - RADIO AIDS - ILS
"Unless otherwise coordinated through Flight Standards, ILS signals to Category I runways are not flight inspected below the point that is 100 feet less than the decision altitude (DA). Guidance signal anomalies may be encountered below this altitude."
Whilst I do agree with the discussion pertaining on AFCS limitations vs approach category, are you still legal in that scenario if, as an example, your CAT I DA is 300 ft AGL ?