A while back motor trend did a study on octane, used several cars, honda, saab, MUSTANG GT... with each car used the recomended fuel (mustang 87). Did dyno test for hp and tq, and a gas milage test. The mustang made less power and got less miles per gallon with 89 and 92 than with 87 (not by much but stll worse). I would not use higher octane gas unless SC or Turbo or other major type of performance mod.
Hope this helps. (remember your cars computer was designed to run with 87)