The 2008 model has completely different engine from the 2005/2006 models - yours is a V-6 engine, the 2005/2006 engines were straight 6 design. Also, 2006 was the last year when MB made non-DPF diesel engines, and these are the two most important reasons why these 2005/2006 are the favorites of MB enthusiasts.
Considering that you can easily get 40 mpg HWY out of this vehicle, I'm not sure there is much that could be done to increase the fuel economy any further. It is the last MB diesel that didn't have the DPF, or any other emissions crap for that matter (with the exception of a catalyst), so the exhaust gas flow is as unrestricted as it can be. I ran across a U-tube channel earlier this year set up by a former MB mechanic who buys old MB vehicles on the cheap and does all sorts of modifications to them. One of the vehicles he bought was this exact machine, and he describes in fair detail the most common pitfalls of these cars, plus how he fixed these issues. He also did some mods to the exhaust (removed the catalyst), got bigger turbo, some gadgets for the transmission, etc. etc. If you want to check out his channel, it is called "legit streetcars". BTW, he didn't have much praise for the V-6 diesels that came in 2007 and thereafter. I'm not sure how active he is nowadays, but I found all his videos about the 2005 E320 CDI very informative. He also lists places from which he is getting all his parts, so if you want to get exactly what he uses in his videos, you don't have to do any searching, he will tell you exactly where to get them and how much these parts will cost you.