I find Scions quite sporty in intent, which is what matters most when sales are concerned.
Enthusiasts will by the 86 regardless of the badge, because to us it's the test drive that'll matter the most. The real $ will come from those buying the car for fashion, and the Toyota badge would be rather harmful to that.
Rehabilitating the Toyota brand perception would require far more than the 86. At the very least they'd need a big dog (Supra) as well, and would do well to consider making RWD saloons based on Lexus IS and GS models. In the end I don't feel it would be a wise business decision. Probably better to attempt to (re)position Scion.