Honestly, I have adapted to drive the car with the lean mixture - therefore I consider it to be fine. most of my friends consider the car to be 'undriveable'. As long as I hold the throttle at a consistent position while cruising and don't mind some hesitation if I get into it to pass someone, it drives fine.
I have 9.2:1 polished rotors (originally 9.4:1) and no overlap (or only stock overlap - like 8 degrees).
.93 L gives better throttle response - hands down, but my research has shown the engine to use considerably less fuel at stoic. than at 0.93L and slightly better mileage at 1.1 L than at stoic.
The efficiency difference is higher at partial throttles because the BMEP is lower, thus the throttle must be more open (lower vacuum). This contributes to better efficiency.
I have :)

I have datalogs of the same stretch of road on the same day at the same speed while varying ignition timing, fueling and ignition spilt. I then used Excel to average the injection time and determine the average fuel economy (for that stretch of road) obtained at each setting. I probably drove that stretch of road 30 times in one evening.

during one of the Rx8 recalls, we got to go drive em around with the laptop hooked up. the IDC/ dealership laptop does logs like the haltech e8/e11's. so i got a chance to go drive an Rx8 and watch what it the timing did.

compared to our "normal" haltech maps, Mazda runs more advance at low rpm cruise (36BTDC L @2500 in 6th, which is like 35mph), but hit the gas, and it'll go to negative numbers, so WOT @2500rpm is like -5BTDC L, this is a taller peak than i'm used to.

decel is where it starts negative split, take your foot off the gas, and the leading timing is -5 BTDC L, and the trailing will stay high and go down with rpm.

at idle leading is almost always -5 BTDC L, and the trailing actually moves around, so it can be like 5-15 BTDC. the trailing stays more advanced than the leading until maybe 1200rpm, or if you give it any throttle whatsoever. ANY load, and the negative split is gone.

WOT over 6k its running 30BTDC L and 15 BTDC T

the second part, i also agree, 25mpg is pretty good for a heavy car like the FC. to substantially improve it, the car needs to be lighter, smaller tires etc etc. the fundamentals of the car matter a lot for mileage, my 58 Tr3 gets nearly 30mpg, and it has nothing on it that could be considered technology, but its a 2000lbs car, with skinny tires, and a decently tuned small displacement low revving engine.

i had pauls Rx8 for months, and the BEST tank of gas was like 21 or something, with 19ish being average, the FC beats that. Rx8 = 3200lbs with 225's

-mike

My thought on this was that we need a compromise of lean AFR with highest HP.

A simple way to do this is use a constant of 20% injector duty (in 5th gear, level ground) and then change the AFR to accomplish highest cruise speed.

This method would avoid reaching diminishing returns by going too lean.

Barry