|if you don't have access to the wiring closet how do you expect to add a router?|
run wires on the floor in the office? You would do the same with a switch
"the corp net was connected to each computer directly and a second ethernet cable was run from each computer to a separate router, that too, would add latency to the corp net?"
No it would not add latency to the corp network since the second nic and router is a separate network.
Again you have no need for routing. What would you be connecting the routers wan port to? The corp network?
You need your pc/macs on the same lan BEFORE it connects to the corp network.
By putting in a switch you get all the benefits of the corp network AND the gigabit speeds you need between the pc and macs.
If you like complex with its more prone to failure add a router. It would have to be a gigabit router or you accomplish nothing. Putting in a router may also prevent you from getting corp updates and in my company would be against corp policy.
Answers are only as good as the information you provide.
How to properly post a question:
Sorry no tech support via PM's