High octane burns colder and needs more compression or timing to fully burn. You'll see higher emissions with that combo too.
Timing is a crutch in this situation. If your cylinder head/piston combo require 36* for regular unleaded to get a clean burn, adding 2 degrees for Premium wont yield ANY additional hp. Match, at best. You didnt change any cylinder pressures with the different fuel. You just lowered combustion temperature.