Well the beauty of the E30 as opposed to newer BMWs is that its put together like legos. Its pretty easy to swap parts out. I don't know what differential you have but if acceleration is not an issue, you can drop in a much taller diff. There is a about a dozen ratios available between 2.79-4.44. I know at least one guy in the US is using a 2.79 with his 1.8 liter M42 16 valve and getting some pretty phenomenal FE in the process. Its just a matter of how much acceleration you're willing to sacrifice.
E30 Differential Ratios - e30tech.com Forums
To reduce parasitic drag you can ditch the PS pump and clutch fan. It was a nice improvement on my M42, swap in Bosch Design 3 injectors (EV6) for better fuel atomization. Right now you have the injector on the left, EV6 on the right. EV6 produces a much finer mist without increasing the total flow.
http://farm3.static.flickr.com/2319/...79b3f801_o.jpg
http://i232.photobucket.com/albums/e...g?t=1243226257
These injectors can be found all over the junkyards for dirt cheap.
I wish they sold the wagon in the US. I had to settle for a 4 door 16 valve.
good luck
Justin