That sounds about right; remember that Ghost has been around a loooong time, since the era when 486's were still pretty common bits of kit, so performance limitations in the server involve failures to cover latency (rare) or otherwise the limiting factor is an I/O channel being saturated (usually in the clients), or there is a limit in the network (usually in the switches).
For an example of latency, the image file data is read by GhostSrv.exe disk using portable code (which means it is using buffered synchronous I/O). If the image data is stored on a high-latency device such as iSCSI network-attached storage, the default OS read-ahead for synchronous file I/O isn't enough to cover this latency perfectly and you do incur a performance penalty. This is fixable, but it's not a common source of problems.
More commonly, in our testing something in the clients gets maxed out (it pays to bear in mind that they are working with uncompressed data, whereas the server side and network get to use compressed image data). IDE hard disks tended to max out at somewhere between 30-60Mb/s of write throughput, and although SATA and SATA2 have a nice high theoretical maximum throughput the hard disks themselves don't tend to be able to reach anywhere close to that in sustained write throughput.
And of course, separate to either is the question of switches. A good switch costs a hefty chunk of change, but if you have a high-quality multicast-aware switch and multicast-aware routers, those are some of the best things to do for Ghost performance full stop. A good Layer3 switch will make sure that image traffic only goes to the parts of the network that need it, which also means that if you're imaging machines on Gigabit paths then you aren't going to have performance limited by flow control backpressure from 100Mb network segments.
I don't know how most customers put their GSS servers; to be honest it's something I'd love to learn more about too, but I assume they simply install it on whatever hardware they already have rather than choosing to provision something separate.