If you have exhausted all your other options, your "less-than-maximum" gas mileage could be caused by an improperly functioning EGR system. My 20R-powered '79 Celica was capable of 33 mpg after changing to a W58 gearbox. However, I had to eventually "delete" the EGR due to a rusted EGR tube (the pump failed soon after, along with the A/C clutch bearing). I noticed the mileage go down to about 29 or so.
One night after I rebuilt the 20R, my bro drove the '79 (with the W50 gearbox pre-detonation) and I drove my '82 with a 22R (w58 gearbox). I could not even begin to touch the '79 in any situation... so to compare a 22R with a W58 (or A40D) to a 20R with a W50 transmission is like comparing apples to oranges.
I'm sure the W50 would stomp the W58, even if the w58 was also behind a 20R, except on the very top end of 5th gear... the W50 would make the engine over-rev before the W58.. I can wrap the speedo needle around past the "mph" on the bottom.. you'd be surprised how stable the car is at MPH... LOL