Perhpas in contact to susanoo it would blow up and be backfired to naruto(cause kakashi said so, since wind feed fire his techniques would power up sasuke's incoming techniques and blow up).
well FRS is not a bouncing ball and susano is not something that bounces back... if so then all other attacks from the previous manga shud be bounced back aswell.. so unlucky for us, that's something which falls out from our theories.. anyways.. since naruto is the one who will be trowing a FRS towards sasuke's Amaterasu Susano mix attack.. it shud hit way harder when FRS hits sasuke since naruto is the one TROWING IT to sasuke.. we all know what kind of damage FRS did against Kakuzu.. even more after it improved we saw how it was against Pain...
ok here's another theory:
When naruto trew the attack we saw how it expanded and cut pain's body into 2 pieces.
Now let's assume naruto trows the FRS towards sasuke and sasuke uses Susano with Amaterasu... If so, the FRS shud expand itself aswell.. and we know that wind makes fire stronger.. and since susano has amaterasu, the amaterasu's fire will ignite the wind of FRS and when it expands it will have WAY more impact.. The FRS could cut susano into 2 pieces or maybe even more damage, and the fire will even boost the FRS in power and area of damage even more..
my conclusion, if this will happen sasuke will get stuck and won't have a way out..