Ok so I got home and tried this for myself.
results:
- Subdirs work and are recognised
- smb mount works and is recognised
- games start from mount
- Slight lags and choppyness from time to time
this was tested with CTR crash team racing rom (PSX)
EDIT: Similar results with NFS both have about the same transfer speed of max 5MB/s "download" to the retropie
EDIT2: No issues with "Metal Gear" and "Tony Hawk 2"
Monitoring the Traffic shows Burst of download Traffic about 250 to 500 KB/s and not much load on CPU or memory for the latter 2... So I guess the lags are chalked up to the emulator ...