Thank you both for the reply.
Gostev, you have made me so happy to know that being able to move backup chains between REFS volumes without rehydration is already on the road-map. With the ReFS synthetic operation 'free space' error being a potential bug, should I log a support ticket for that?
dellock6, I found the blockstat utility very very quickly after running the first customer billing report on the new backup system (Veeam) using our existing reporting module - the numbers were so much higher than before I thought I was going to be rich... no, I didn't think that actually, rather, I was stunned like a Deer in headlights. The ability to see 'actual size' of files on disk wasn't even on my radar as a potential shortcoming. When we tested Veeam using REFS we did that internally, on our own servers and we don't bill ourselves so I never ran the billing report generator against the files on disk.... oh boy that was a huge mistake... So I did some google searching and found blockstat. Hallelujah, right? I had a go at using it myself to generate size on disk figures and what I got out of it didn't add up. So I handed the blockstat utility to our lead programmer to dissect and make work for me. Started by doing blockstat against all files per-VM per-extent. Then tried querying Veeam api what files it had fast-cloned together and run blockstat against those files. No matter which way we tried to calculate the size on disk it was always inaccurate. For a single VM the numbers look reasonable. When we calculate the size on disk for all VM's and add them together, they don't match the actual disk consumption (as reported in Disk Management). Some days, on some extents, the numbers were "close enough" but other days, or on other extents, the numbers varied wildly. We ended up giving up on this and hoping that one day Microsoft would eventually build this capability into Explorer (cmon MS, block-clone has been around almost 18 months now!).
If anyone has been able to get accurate figures using blockstat I would really appreciate if you shared your method or code!