Allen,
Thank you for the suggestions. 

By 'proxy agent' I mean they are authorized to do backups on behalf of the 
target server. 

We are doing possibility one, in your set of cases, with four agents. I kept 
the example simple for readability, but perhaps some clarity was lost.

It seems we may just need to endure slow backups until the entire filesystem 
has been copied again by the proxy agents.

You say,  "I will note that this is unlikely to accelerate your wall-clock time;
if you've got resourceutilization 10, you've probably got 5+ threads
walking the FS, you've probably moved your bottleneck to IOPS on your
NAS as it tries to pull the metadata to satisfy the FS walk.  20
threads won't do that faster."

Are you saying that reducing Resourceutilization would likely improve the 
throughput of the backup? Or, that the "backup by proxy plan itself is ill 
conceived for some other reason?

Thank you,
Keith