As an aside, it’s really more about latency than throughput; especially when you are looking at gapless playback issues.
I just moved my library from a WD MyCloud 3TB onto a Firewire 800 direct-attached disc on my 2011 Mac Mini and my occasional gapless playback issues disappeared.
Funny thing is: both my NAS share and the DAS HDD have very similar numbers with Blackmagic Speed Test. But NAS always suffers from greater latency in the initial read request - there’s more protocol overhead and the target has to react to the request, from an operating system standpoint. This is the case even with higher-end NAS devices, in my experience.
I never have these problems with MPD playback, using a mounted NAS share, and it’s my belief that MPD is more aggressive in pre-loading the next track to make gapless playback completely smooth.
So, it’s not that Roon needs more buffering, it’s that it needs to request it earlier in the playback process.