What's your definition of "stable"? Our departures are independent and diverging so nothing
should go wrong. Departure climbs one above the other and crosses it over. No real drama and you'd be surprised at how little space you need to do it. We don't have the numerous pilot navigated sids like in the UK. All jets from the same runway fly the same sid, in trail spaced by the tower and vectored on course by departure.
Like I said earlier, Cat3 is rare here, maybe less than 20 hours/year so not many airports have developed Cat3 infrastructure and not many carriers are Cat3 capable. Cat 2 is more common and spacing requirements are reduced for that and capacity is better.
I wasn't involved in the request but the guy who was seemed surprised that there was so much of a difference in criteria and that TC wouldn't entertain any reduction. Maybe because of the rarity of the event, TC are reluctant to make a decision.
Or Cat 3 runways are 05 ad 06L and usually in low vis the wind is from the south! Makes for interesting times in tower when in mixed mode: ground speed 200kts over the FAF
Segregated operations are not only more capacity friendly, they make for a much easier time for tower and ground.
DC