In the UK, personnel in this sense is most likely to be considered as a plural noun.
If you are referring specifically to the personnel department, that could well be different. We (BE-speakers) might very well refer to the department as a single entity - personnel (singular).
From my recollection of those threads I would expect AE-speakers to consider personnel singular in almost all cases - but they would need to comment.
Enough of generalisation.
If faced with the target sentence, I would find it extremely difficult to consider personnel as singular. What on earth would one say?
Personnel is not used to the best of its ability (BE eek)