Thanks for your reply.
I can confirm it, that the most effective 'mod' is a lighter right foot + other eco-drivingstyles.
However I'm seeking for additional and additional solutions... ;-)
Don, I drove my 2000 Jetta TDI 5-speed weekends, commute to work and on the job. It had 335k miles on it when I gave it to my son in 2013.
I drove conservatively (speed limit) and took advantage of "coasting" in gear. The ECU shuts-down the fuel to zero injection when coasting in gear down to about 1000 RPMs. A good example is knowing where the Stop Signs are, the traffic lights and down hills are in your route. Use the coasting in gear method when you approach them.
I did an Excel Spreadsheet during those 335k miles. I entered practically everything from fuel to windshield washer fluid.
It averaged slightly over 51 MPG. Now at 390k miles, my son (who drives it different that I did) says he gets 48-49 MPGs. The engine is bone stock and no tune. I was driving 36k miles per year when I retired in 2008. I changed the oil every 10k miles and the fuel filter in late fall just before going into the winter months...... never a fuel issue. Oh, and the stock air box/filter is better than anything you can replace it with.
But, I'd say the best mod you could do is a Tune. A Stage 1 tune is all the stock ALH needs. I got a Stage 1 tune for the '02 ALH engine in my 84 Vanagon. The Tune made a big difference in power and hasn't hurt the MPGs as long as I drive it sensibly.