We have a client that has asked us to do similar things several times – 8 linked hi def projectors, 36 flat screens etc etc. We’ve found the best way to get through the approval process to the final renders is to set up a master comp, as you say, from a template/plan of the screen layout.
We usually position colored solids, numbered to represent the different screens. We then make comps for each screen and import the master template into each of them, positioning it in the correct place for each screen. (The colored solids help you align it in each comp with accuracy). Then, all the work is done in the big master comp.
When it’s time to render, you can render the big master out (at a reduced size) for client approvals (this helps everyone get an idea of what the actual thing will look like) and then the finals are rendered from the individual comps.
Render times, as always, depend on what you’ve got going on, but we’ve never had any real problems.
Good luck!
Nick