While automatics are rare, there are still a lot out there. The big question is, what are your STFT and LTFT numbers like in the low RPM's. I noticed my MAF scale was too lean, and consistently had LTFT's over 10. Since scaling my MAF closer and closer to zero STFT and LTFT, I've noticed the car get quieter and quicker.
Automatics also have much higher gear ratios than manuals, so a change in final drive will give much better pickup in the lower gears, while still getting decent economy in 6th gear. I just ordered the entire kit, bearings, seals, shims, solid spacer, and etc. from Spence @ Weir Performance. Here's the price I'm paying for all the parts;
FRS/BRZ 4.88 final $268.70 includes pinion depth shims
Install kit $313.94 (includes all diff bearings, solid spacer with shims, axle seals, pinion seal, pinion nut, gear marker)
ship $20.55
Total $603.19
Installation should be about half that, so roughly $900 - $1000 for everything replaced with new parts.
Stock gearing with the 4:10 rear end is 0.582 @ 4.10 = 1954 RPM's @ 60MPH. Gearing with the 4:88 is 0.582 @ 4.88 = 2326 RPM's @ 60MPH. So, I'm not losing that much MPG, but much lower gearing in 1st, 2nd, and 3rd. 6th gear is still taller than a manual's. I would have to change to a mythical 5:404 ratio to match the 6th gear of a manual. You could go to a 5:29 and still have a taller 6th gear than a manual. The gearing change is the best thing you can do for your automatic in terms of low-speed performance.
I also recommend getting a set of used strut/spring/tophats from a BRZ. Then you can have a comfortable setup with GT86 front suspension and BRZ rear suspension, or a firm setup with BRZ front suspension and GT86 rear suspension.
