| ...they will figure out how to make the pipe bigger.
Yikes, your IT people think networks are made of pipes?
Seriously though, you should definitely do some modeling of bandwidth across your network structure; you will have to estimate bandwidth anyway when you plan for how many cameras to assign to each archiver. Plan for the future, not the present.
Since you have cameras already in place, hopefully your IT folks are familiar with the network load that these cameras will impose and are not just assuming that there will be negligible load - cameras can use a lot of bandwidth, especially hundreds of them.
As someone mentioned, get Genetec involved in the discussion; they do validate network designs and you (and your integrator if you're the end user) should be going over all this with them.
Are your clients on the same VLAN? From your description, I am assuming not. If your clients are on the same VLAN, then hopefully your network switches support IGMP snooping; if so you are all set. If you have clients on different VLANs then you will need IT support to actually route multicast - it's 90% understanding, 10% implementation.
With Security Center, multicast will benefit you only when you have multiple people viewing the same live streams at the same time. Most importantly, this will reduce the bandwidth on your archive servers (servers that manage and record cameras) and quite possibly will be the determining factor in how many archivers you need.
On a multicast capable LAN, Security Center controls multicast streams for cameras by either directing the camera to send the multicast stream (for those cameras / encoders capable) or by having the archiver itself multicast the stream if the camera is configured for unicast or incapable of multicast itself. If multicast completely disabled I believe that it is the archivers, not the cameras, that send live video to clients. I could be wrong on this as I have not worked with SC in a unicast only environment.
A few other notes on multicast in Security Center: SC turns multicast off when there are no live viewers, switching back to a unicast stream when only recording; this can save on multicast addresses. SC also allows you to indicate different capabilities for different LANs, so you can configure your video LAN for multicast but have all external LANs be unicast only.