You're not getting any additional performance running 89 in a vehicle tuned for 87, other than making your load a little lighter with a thinner wallet. High octane does not mean more power, it just means higher resistance to detonation. You can make more power in a car tuned for 91 because you can build the engine to run at a higher compression but simply adding higher octane to a motor built and tuned to run best on 87 does absolutely nothing helpful.
I regularly tow a 3000lb boat in the summer with our CX9 and it does ok as long as you shift it manually. Left on it's own in drive the automatic will constantly grab 6th gear and then hunt in and out of torque converter lockup and sometimes between 5th and 6th gear. That's a guaranteed recipe for cooking a transmission. It will pull flatland and slight inclines at 65-70 in 5th no problem. My only issue is when you shift it manually on a long trip being accustomed to automatics you'll forget you're in manual mode, slow down for some traffic and when you go to speed back up the trans doesn't kick down. It would be a much more capable tow vehicle if they simply gave you a tow/haul button that would lock out 6th gear and remap the shift points.
Also, don't forget that this same exact vehicle (engine, trans, cooling system) is rated for 4409lbs towing (2000kg) in other countries. While I'm not recommending exceeding it's 3500lb rating here I wouldn't be concerned about towing up to our 3500lb rating at all.