6.2.0 has no manual to download

I think it depends a lot on the pattern of I/O to those files. If it's sequential write and random reads, then it really shouldn't be a problem. But if those large files were undergoung random read/writes or were growing slowly over time -- such as might happen with a multi-GB VM disk file -- then turning off CoW might yield real performance benefits. I don't tend to run VMs from NAS storage, but on my Linux workstation running BtRFS, I disabled CoW on the folder tree containing my VMs for exactly this reason. For shares where I store large bitorrented files, which aren't modified but do receive random read activity, I wouldn't worry one bit about having CoW enabled.