[Padgett]

It depends on the fuel and advance maps whether premium will help. My Jeep is a flexfuel so has the maps that would accomodate premium but an engine designed for 87 may not run as well.

Gasoline has the same btu/pound (power) regardless of octane. The purpose of high octane is to avoid detonation which is not an issue with direct injection. However to avoid detonation 93 also burns slower than 87 which means for best power/economy it needs more advance (back in the day we used to bump the timing about 6-8 degrees when running premium (can see it in the manufacturer's specs back then) to accomodate for the slower burn.

Today all of that is handled by the computer but an engine designed for 87 may not provide enough advance for 93 and you have no way to adjust it. If so both power and mpg will get worse, not better.

Could tell by monitoring both with Torque Pro and by watching the advance but is hard to tell without proper instrumentation. As a consequence it is best to follow the manufacturers recommendations unless you buy a tune for something different.