I also find it interesting that your ops manual states that you do not require the chart-stipulated (adjusted for other things HIAL U/S etc) visibility, just what you deem "adequate"
I know that wasn't directed at me but I thought it needed clarifying anyway. Re-read my bit above and you'll see that you don't need the visibility (adjusted for U/S equipment etc) at DA; you need it BEFORE you get to DA. If you don't have it before you get to the Approach Ban Point you cannot continue the approach, so you'll never get to DA.
The required elements required to be seen at DA depend on the type of ILS. CAT 1 has some fairly high requirements e.g. PAPIs OR Rwy markings OR edge lights OR Rwy end lights etc etc but has a high DA and, therefore, you are likely to see them. CAT 2 has a lesser requirement but includes a 'lateral element of the approach lighting system' to assist in your adjustment of line-up. CAT 3A has only a 'one light' requirement but you've got lots of back-up equipment being used now (fail operational etc) and there's no need for you to see much 'cos either the autopilot or HUD/Flight Directors are doing it for you and, almost certainly, you're going to autoland anyway. Cat 3B you're along for the ride and employed to make sure the kit behaves itself.
SO, in summary, you do have to allow for equipment being U/S and this will 'up' the visibility requirements. Not at DA but, more than likely, at the descent briefing.