OK,
So I have an Odroid XU4 as my NAS running headless KODI.
My HDD’s are plugged into this and shared via SMB.
I want the KODI instance to use the local paths (/media/virtual/Movies) when scanning / exporting fpr a lot faster speeds.
But, when added into the MySQL database, I want them added as SMB paths which are smb://192.168.20.3/Movies
So, i thought I could keep all the sources etc as smb paths but just have the below on the headless instance.
<pathsubstitution>
<substitute>
<from>smb://192.168.20.3/</from>
<to>/media/virtual/</to>
</substitute>
</pathsubstitution>
But, doesn’t seem to work.
As far as I can tell, KODI still is using the smb paths when scanning.
I think path subs might only be used for playback?
The other way is have the local paths in the MySQL database but then I need path subs for all the other KODI instances.
Update:
Sorry, I was being silly.
It does indeed work.
I changed the <to> path to a non existent path and the update scan failed.
I then timed the scan with and without the substitution and it was 2:30 with and 10:10 without.
It also correctly adds smb paths to the MySQL database 
PERFECT!