It doesn't work like that. The distance that wireless AC can reach is the same as wireless N. That's because they use the same 5 GHz channels range.
A router running in b/g/n mode (Mode 2 in the case of your BT Hub) radiates signals out in all directions equally. Where the signal has clear line of sight to the thing it's talking to, then you'll get the most range. However, as the range increases then the signal strength falls off, and as it does then the amount of errors in the signal increase which means more error correction happening, and this reduces the signal speed. In practical terms, you can stream HD video when close to the router, but as you get further away then the signal starts to stutter and eventually stops.
Things that affect signal strength (and hence speed) are solid walls, metal (think 'foil backed in-wall insulation'), metal framework used to make stud walls, water, and high density building materials such as concrete.
When your router is running in Mode 1 (g/n/ac) then it's still radiating signals all round for g/n, but when it's talking to an ac device then the aerials are able to concentrate more signal power in that direction. Since signal power = data speed, then the effect is the ability to stream HD video at a distance that b/g/n can't manage.
What ac can't do is give you a Wi-Fi signal say at the bottom end of the garden or in a room in the house that b/g/n alone couldn't reach.
In summary, the range of your router won't change. Any ac devices might experience a drop in the data speed. You're just going to have to try it out to see if it fixes the laptop stutter issue without adversely affecting the speed to any ac devices in the house.
Don't despair if one of the kids complains that they can't watch Youtube vids in bed any more. This isn't the final point in your journey. It's possible to upgrade a laptop to ac with either a plug-in USB Wi-Fi ac dongle, or even by changing the wireless card in the laptop itself on some models.
Another alternative is to switch to a NAS drive for your home's music library.
If you haven't come across NAS drives before, then NAS stands for Network Attached Storage. In essence, it's a hard drive in a box with a mains power supply, and it can be read from without the need to be attached to a PC. The boxes themselves are relatively small - typically about the size of two paperback novels back-to-back. They're virtually silent, and connect with an Ethernet cable to one of the spare ports on the house router.
Set-up is fairly simple. Once up and running, you'll have what's called a shared folder which is where a copy of your music library will live. The Sonos can be instructed to look there in a similar way to how it looks for your laptop drives. The difference with a NAS is the music library is always available to the Sonos and any other devices you decide to set up to access it. This means no-one needs to fire up a laptop just to listen to the library. A side bonus is that NAS drives are a lot more economical on power.
