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"Drive-by listing"? A lot of buildings got listed years ago with a cursory inspection from a passing car hence the term.
The DG units I think you're referring to are slimline. They have a smaller gap which is compensated for by using 3 gases which have better insulating properties than the standard gas and the glass treatment reflects something (UV maybe) that improves their U value. They were far more expensive 10 years ago but they're now available from most glazing suppliers and they cost more but not an extraordinary amount more than normal units. Manufacturing quality of normal units is very variable and often the more unusual options are only manufactured by the higher quality manufacturers and cost more. It's very difficult to break a toughened double glazed pane, much more likely that the seal will fail and you get moisture in them before you break the glass.
I've been looking into this kind of window conversion using slimline units but it's not that economical for a lot of the sashes I work on as they have up to 18 panes in the upper sash. It increases the cost dramatically as the units have a minimum manufacturing cost. It also doesn't have a massive acoustic improvement as the gap is important in cutting down the sound waves through reflection.
The other problem with DG acoustics is once they are open they're not much better than normal sashes, and they are usually open in summer when your pissed neighbours are staggering back at 2am or there's a big party going on over the road. If you live next to a busy road then I feel sorry for you but heavy curtains will still give you a better nights sleep (even with double glazing).
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Airhead
I have double glazed sash windows, with some fancy coating and gas filled. They used the original wooden frames so you can't even tell they are double glazed (my building is semi protected because it's some sort of poor-mans-listed). Think they are around 10 years old now and no issues, I can try and dig the companies details up... although I have a feeling they were pretty expensive (I live in permanent fear of breaking one), the previous owner quite proudly mentioned the company was on Grand Designs.