Hello, I as well have a 1985 300D-T and it gets about 23 mpg. Every once in a while I might get 26 out of it, but only when I am really gentle on it, and I consider that mileage pretty good considering the published mileage is 21 city 22 highway.
I bought the car 18 months ago with 132K on it, and it now has 162K and the mileage and reliability have been very consistent. I got the trap replaced for free, as you should, did what you did with the EGR and Bypass (which was leaking causing all sorts of vacuum related problems) and bypassed the electronic overboost valve on the firewall beside the brake power booster. I also got rid of all the assorted vacuum related valves that were controlling the EGR and Bypass, which were locvated beside the power steering pump, and that cleaned up the engine area pretty good.
I understand your concern, as I bought my diesel thinking it would get about thirty mpg, but only after investigation realized the best it would do is what I mentioned. I do all my own work and still have put over 4,000 into this car in parts I have bought on-line (all-OEM stuff). I try to resist the urge to sell it to get a 190D, but that fuel mileage for a car I want to keep wouls sure add up over time.
Good Luck, Adam