I curently have 10,300 miles on the clock and until the the last 2 tanks ran nothing but 87. I noticed a 2mpg increase and an increase in accelertion that could be felt. I do almost all highway driving as I live in a rural area. I will continue to use 89 octane gas.
Unless you are towing or heavily loaded, you are better off with regular. I did an experiment a couple weeks back and used premium one direction (400 miles each way, all flat terrain) and regular coming back. Assuming some small errors with pump shutoffs, how much was left in the tank when filled, a small detour going there, etc., the mpg was virtually identical both ways (20.5 vs. 20.9). There are more details in a thread I started about two weeks ago.
I agree there is no advantage to using premium as the engine does not have a high enough compression ratio to take advange of premium fuel but 89 is a different story. Since I get 2 mpg using 89 octane I get better performance and saves me money. Example 320 miles @ 16MPG uses 20 gal. 20 gal @ $2.60 = $ 52.00 18 mpg for 320 miles= 17.7 gal $ 2.70x 18=$ 48.80. A savings according to my math.
Using a higher octane isn't required. If you notice a difference then great but mostly you will not. Along with the compression, the ignition timing is limited so to benefit the higher octane you would have to have your PCM reprogramed to take advantage of the higher octane.
I installed a "tuned" PCM in my 02 Silverado and it made a tremendous difference in performance and an increase in economy. Not to much but some considering the increase in performance. I need to get in touch with Wait4meperformance and see if they will be upgrading the Acadia's pcm.