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 ...