turbo87

I have 2014 c7 with ECS everything is stock other than ECS. If i install the small restrictor plate with a new tune can i use 91octane? or do i need to change pulley? Now i have the large plate and meth but the tuner's can't seem to get it right. Talked to new tuner today but he wasn't for sure if the small plate would be to much boost with 91 unless i installed headers to lower boost?? Then he also said change to E85 but. very few pumps around here.