I looked into them a year and a half ago. There were about 3-4 overdrives on the market for the Defender. I asked Ritters (in Melb) for a quote on one, as they were the local agent. They said, don't bother, wait awhile and get a 6 speed box from a 2008 Defender. Someone at the time mentioned there was a company in the UK working on a bell housing adaptor (or similar) to fit the 6 speed to the 300Tdi. Even if I were to do this, I'd wait a couple of years to see how the 6 speeds fare.
I've got an overdrive on the SIII and it helps with highway cruising. The 2.25 will sit on 110kph all day (on the flat) and I can still talk to my passanger (there's also a lot of sound proofing in it).
No difference to economy though, I guess pushing an aerodynamic (not) brick that bit faster would negate the advantages.
