Seeing as how they are no longer taking orders for cars... I don't think you'll see sales slipping. Remember, this car is a low-volume line. And most don't stay on the lot longer than a couple days before sold. Especially if they are 6MT.
They do have a turbo version on this engine out, but I'm skeptical they are going to bring one to market any time soon. Even better if they do. I'll sell mine and buy one of those!
Subaru has only built 3000 BRZs worldwide. And over 8000 GT86/FRS. With such limited number, they would have to add an entirely new line for turbo models. If it were a vehicle they could sell in high enough volumes, they might do that. They did, after all, bring the STI to market in 2004.