Here’s the deal: if you use 87 octane your engine will try to compensate by retarding your ignition timing, which will effectively limit the power you can get out of the gas, and you also run the very real risk of damaging your engine. Expensive damage. Hole in the pistons damage.
Said differently, you’ll actually get more power out o the 91 octane because your engine won’t run right on 87 octane…and may just self-destruct.
Do yourself a favor. Use 91 or better.