In the 5 months I've owned my GV70, I have experimented 2 months each with 87 octane, 89 octane and now currently trying 91- 93 octane. I really can't say that I noticed much of a diff btw 87 and 89 but that 89 seemed to burn less quickly I think. Because i have been refilling my tank at half-way, the initial change to the higher octane results have been mixed a month in. I am however starting to sense that, after a month, that the higher octane seems to be running more efficiently while also burning slower. It might even be slightly 'peppier' in sport mode.
Once I finish testing the highest octane, I'm going back to reg for a direct comparison of 87/93 while it's fresh in my mind.