Is it the network bandwidth or actually the print server function causing the issue? I'd be looking at the print server, especially if the files are very large. Is the USB server capable of properly buffering the data and note it may only be operating at 10meg speed?
We run a SBS2003 network with some 19 users. Only in the last few months have I upgraded our switches to gigabit, before then everything was at 100meg speeds. I never saw a bandwidth issue. In fact our real slowdown was due to a database system that the workstations can't really handle. Even so we have 7 or so printers, all of which are ethernet connected with the server doing all the active directory stuff for those. Only one printer is USB and that is hung directly off a workstation so the workstation would act as the print server even though that printer is then shared across the network.
Can you try hanging the printer directly off a workstation, or even the server itself and get rid of the USB print server? If that proves to be the issue then look at getting a printer with an integrated network card.
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How do you ensure the data traffic is going through to the correct router i.e. netgear firewall or the VOIP thingy. Is there a chance that stuff is bouncing though one before it hits the correct one? What is actually controlling the QoS / load balancing routing rules?
I'm not sure from your diagram. Is the top row of connectors a patch panel as I can't see why you would have a connector to that as well as the switch
SBS2011 is different to my SBS2003 setup as our server has two network connector, one to the Internet, the other to the office network switch, so our server does the routing, DHCP and firewall stuff for the workstations. Perhaps with the data flowing around your network, especially the added VOIP traffic, an upgrade of you swithc to gigabit might add extra capacity?