I would recommend against a hand-rolled distributed filesystem arrangement with different people accessing different drives for the files because you will hit significant sync issues when two people modify the same file in different places.
There are a number of distributed filesystems (and distributed block device arrangements you can build filesystems on) out there (see https://stackoverflow.com/questions/269179/best-distributed-filesystem-for-commodity-linux-storage-farm for a could of pointers to open source solutions). These are geared towards fault tolerance, size scalability, and remote access performance, but they will by nature improve multi-client access scalability too if you have multiple local nodes. Unfortunately you are not going to be able to use these in off-the-self not-enterprise-targeted NAS boxes (at least not with any NAS box I've seen).
A speed limit between 50 and 100Mb/s with a gbit network adaptor is likely to be imposed by either the network interface or the I/O controller that is managing the RAID, but your key bottleneck for throughput is likely to be the drives rather than either of those. As several people are pulling data off the drives, the heads will be bouncing all over the place, getting a bit of data for client 1 from here, moving to get a bit from client 2 from there, then client 3, then ..., then back to servicing client 2's request, ..., ..., ... - and during each head move the drive is not able to transfer data. The IO controller can reduce this effect using a few tricks (clever elevator algorithms fro each drive (see http://en.wikipedia.org/wiki/Elevator_algorithm), clever sequencing of requests between drives when the requested data is in more then one place, and so on) but there is a limit to how much help these methods can be.
If the NAS box has its own RAM for caching rather than relying on the drives to do all this, you might find it has options somewhere for controlling read-head and read sequencing which can help by further reducing the amount of head movement required to service the same set of concurrent requests (though be careful to test any changes thoroughly as you can make things worse rather than better and the best options for one access pattern could be atrocious for others).
Another option to look into is SSDs as these negate much of the latency that bogs down spinning-metal based drives in random- or multiple- access situations. While it will likely be far too expensive to replace all your space with shiny new SSD technology there are half-way options. Hybrid drives are spinning-metal with a chunk of SSD storage built in that it uses as cache (and being non-volatile they can use it buffer write operations as well as speeding reads). Some NAS boxes have built-in support for using SSD drives as a large non-volatile cache for the larger traditional drives which removes the need to replace all the drives with hybrids and would likely be more be more efficient, though IIRC this feature is only on more expensive boxes at the moment.