There is no real performance value to the client upgrade. But, in my case, as you can see in the post above, the way flags are handled can have a pretty big difference. Using the v4 client with the -forceasm flag turned on 3DNow optimizations which slowed that cruncher down a lot. By using the -forceSSE flag (which works now for whatever reason), brought the frame times back down to the same level as when I used the 3.25 client.
Usually, a new client is mostly to fix minor bugs in previous clients, and add a few useful functions. Such as the -configonly flag that stops the client and exits after configuration, useful when someone wants to set it up as a service.