This should be straightforward - just use 12.5mm plasterboard sheets and rely on the 300mm of rockwool over the top to provide reasonable soundproofing.

However I'm a bit noise sensitive and like to over-engineer, especially when blocking out noise pollution.

The new highly ventilated soffits will permit far more noise into the attic space than previously.
e.g.
Overhead, there recently seem to be far more airliners flying at lower altitude and climbing.
Pidgeons in plague-like numbers - attracted by the crazy (elderly) neighbour's indiscriminate feeding.
. . . the occasional, but very loud yoof parties, in houses surprisingly far away.
etc.

In extremis I could fit 15mm SoundBloc plasterboard on Resilient bars, but that would have to interface with the existing fixed plasterboard anyway.
Is there any point in mixing high spec noise-blocking ceiling components with the adjacent existing conventional ceiling ??
. . . . .
PS. The new window and door frames have the compulsory trickle vents.

>> Depends on the areas involved to some extent though the conventional ceiling will greatly limit any improvement. Is there any chance of adding another layer of plasterboard to that? <<

Yes, by knocking down the old exterior back wall, an internal dividing wall and removing doorways connecting to hallway I have to match the levels of all the existing ceilings.

One of the original rooms has a double boarded ceiling i.e. 9.5mm * 2.
I guess that this room was intended to be a bedroom - in the days before loft insulation - to reduce noise via the attic.

This (lowest) ceiling now defines the level for them all. Hence the existing kitchen ceiling and hallway have to be augmented with a second layer of 9.5mm to match.

Perhaps an equivalent sound performance for the new extension ceiling would be 15mm Soundbloc or double 9.5mm ordinary plasterboard.
[ I think I'll forget about the resilient bars : it seems to be spooking the tradespeople !! ]

Posted By: djhThere's definitely an art to them. You have to learn which screws work best with what driver and how much pressure is best to get the point through the bar without bending it away. I'd suggest trying a sample area if they're nervous; they're not too bad once you get used to them.

What screws do you use with them?

Posted By: DarylP....self piercing, fine threaded.
The knack is to make sure you screw through to avoid the joists carrying the bars.
They do work, if installed properly.

In our case, specifically collated Hilti screws with a Hilti gun. We had trouble with other brands of either. In some places we also had trouble with the fine thread for some reason and used coarse thread. Using the correct length is the key to missing the studwork and joists, so they go through the bars but don't reach the studs.