This topic is off-topic for this forum.
There are many things that could be causing whatever performance problems you're seeing. You should work on identifying more specifically what the problematic behavior is and try to find the bottleneck(s) before spending $ on new hardware.
There might be too many clients accessing too many files at the same time and the NAS can't keep up. Maybe you need to add link aggregation on the NAS to spread the network load across multiple ethernet links. Maybe your NAS can pump out 50-60 MB/s or more if it's transferring very large continuous files vs small files. Maybe it can even go faster but the clients you're copying to are limited in their write speed (or read speed if writing to the NAS). Maybe performance was okay when the NAS boxes were new and now there's some kind of problem with the physical disks or the filesystem is badly fragmented. Maybe the RAID configuration is not optimal. Maybe you're using SMB and you might get better performance with AFP or NFS (or SMB might be better... not picking on SMB here). Maybe there's some TCP tuning that could be done to improve the situation, or enabling jumbo frames might help.
Maybe buying new storage would help. Maybe not.
I wouldn't recommend buying anything until the problem is understood. If you're not sure how to proceed, I recommend working with a consultant. One place to find a consultant who knows Macs is http://consultants.apple.com.
I really appreciate your reply.
However let me say that we will change the NAS and the switches or anything else. But preferably not the wiring (recent and CAT5e) unless necessary.
The (modified) question is: Will Xsan give me 50/60 MB/s across 35 metres of CAT5e (assume no other traffic)?
If this is possible, please tell me what I need to buy.
If not please tell me to dream on and go Fibre.
Thanks all, really appreciate your input.
Xsan does not function on ethernet (only the metadata travels on that, the rest requires fibre channel).
So, why not trade out your switch first. That D-Link is probably a really crappy switch with a fabric rated way below the aggregate port total. When buying a switch, make sure that the fabric exceeds the aggregate total of ports. For example, if you have a 48 port 1000Base switch, it should have a switching fabric that meets or exceeds 48000. If you do not, then you have a fabric that chokes under heavy load.
If your Cat5e have been tested and validated for 1000base traffic, you should be getting around 4 GB/min of transfer (depending on file sizes, number of files, etc). But a good rule of thumb for 1000Base is to hit around 4 GB/min of transfer. And this is without shaping the connection via MTU or other sysctl tweaks.
I would start with a really good switch.