i'm in the same boat as you dave, i still see pretty bad milage with my 94 5sp loyale, but i've yet to have a good highway only run on a tank. i almost did, but that was with full AC and the 15 miles i didn't spend on the highway were spend sitting in a line that took 2.5 hours to get in to, and 2 hours to get out of.
what is your timing set at? when i got it it was at 29btc and it was moved to 20 when it was smogged, but then it had a horrible flat spot at 2500rpm. so i wasn't sure if the tank i used at that setting would be any indication for mileage -- and that tank had no AC since it wasn't working yet. i reset the timing and it runs better, but still kinda flat at low rpms and i'm only getting like 280 miles out of 12 gallons.
i looked at the airfilter today and it looks pretty good and it doesn't sound like the plugs/wires upgrade you did helped much. anyone got any suggestions to try?