The FA family was derived from Subaru's FB family;
Derived yes, but share close to 15% of the parts per Subaru. The FB and FA engine share almost nothing except a block. All the internals are different. Maybe some bearings are the same but I don't have time to tear down the engine part by part.
The turbo motor, the FA20F was developed first (as in before the FA20 found in the 86).
Wrong. Subaru was having a problem with getting direct fuel injection technology working and as part of the GT86 deal Toyota offered their own DI and port injection system to help smooth the deal over, since Subaru was otherwise getting fucked proper. This is part of why Subaru is allowed to sell 1 BRZ to Toyota's 5 86's or whatever it was.
The FA20 D-4S and the FA20DIT (you keep calling it the FA20F, which is only used in the EUDM, btw. not the USDM.) were co-developed with the lead being on the D4S system having the edge, the 86 released in 2012+, and the Legacy released in 2012+ were released within months of each other, This is evidenced by news articles from the time period mentioning that the sweet DIT engine that was released in Japan won't be getting put into the 86 (sad trombone).
we didn't get the FA20DIT in our cars until recently, our leggies, OBs, and fozzies used the EJ253 and the EZ36 respectively, with the XT Fozzy making a reappearance coinciding with the release of the WRX equipped FA20DIT). I believe the engine choices are still the same in USDM
FA20F, out of the Legacy, Forester and WRX... like I have already said. That was developed slightly later than the D4S.
manufactured and sold in much greater numbers, and sold in a WRX which is the same price as the BRZ, but with the said turbo motor, AWD (more money), better brakes, tires, and better standard equipment would be more money?
The 86 uses parts from the lower end of the Subaru and Toyota parts bin. You have a ZC chassis that was developed from the ground up (actually a shortened impreza, but for all intents and purposes, new), using the cheapest parts both had. The axles for example are the weaker Impreza non-turbo axles, Not the STI axles which are made better. These are also the same parts that break when you add more power. Up rating everything to support the extra power would make the car more expensive than it is. I'm not talking doubling the price, but it would be on par with the WRX, and at that point, you have the Mustang, Camaro, WRX, Genesis Coupe, Golf TDI (R if you go to BRZ limited optioned out levels of cash), Focus ST, (more recently the RS), and the Fiesta ST to contend with. Most of them making better power.
In the same price point currently, you have the Miata, and a base Mustang.
The only reason why the WRX and STI are as cheap as they are, is that they haven't made many real mechanical changes to the models in roughly 10 years. I mean, the EJ25* engine was in use in 2005, and it's STILL in use in 2016 in the STI. The R&D was done 10 years ago on the EJ engine, and we're just FINALLY getting over them. Price increases are minimal, because they've made their money back on it. The BRZ? it was a new platform, R&D is expensive, and they're not making much on this deal.