So I moved my home from being alongside root in subvolid=0 to being under root instead to have a play around and work out some pros and cons of having home a subvolume under root with no separate mount ...
Quickest gain is cp --reflink works all over now ... I can have the same extents for ISOs in my download directory as my libvirt images saving space and copies out of home to elsewhere are instant...
On the downside with home under rather than alongside root if a snapshot of root is taken and then switched over to in order to revert something then home would be 'lost' without it too being snapped over under the new root. Although snapping and reverting root like this is an advanced operation the resultant behaviour is quite unintuitive.
There is a work around to my x-dev reflink issue that kicked this off of course - just mount subvolid=0 somewhere or other (say /mnt/rootvol) and within that root and home would exist on the same device allowing the cp --reflink to work ... which would then be seen back under the normal / and /home ... but without knowing a fair bit of how btrfs works this would be pretty hidden and unintuitive to the general user.
Having home as a separate mount in the same pool has other benefits as well of course such as being able to use different mount options...
So now I feel I'm at somewhat of an impasse as to which works out best overall and whether to solve the major cons it'd be better to have recursive snapshot of subvolumes in btrfs-progs or whether cp (or rather the IOCTLs called in the kernel) should recognise subvolumes mounted separately from the same pool and carry out reflink ... or possibly bugs both ways as useful enhancements?
Either way I'm not really sure what the optimum anaconda behavior for a BTRFS layout would be ...