Not really. You won't see any LAN transfer speed increases.
It depends, I have a DIR 655 with my DNS 323 and desktop PC all running 1Gbit and I have enabled Jumbo Frames at 9000 on both.
Since I transfer multi gigabyte files I got significantly better read performance from the DNS 323. Before enabling Jumbo Frames I was getting 12MB/s write and 15MB/s read. With 9000 Jumbo Frames enabled I'm getting 20MB/s write and 30MB/s read from my DNS 323. Also when a mate of mine brought his desktop PC around we were transferring at 60MB/s between our two desktop PCs so the DNS 323 must be bottlenecked at 30MB/s.
I must admit that small file transfers I saw absolutely no improvement. So it depends on what you transfer across your network and the speed of the source and destination, on whether or not you get better performance from enabling Jumbo Frames.