Excessive detonation could damage the cylinders but 87 PON Unleaded should not bother the cats at all.

My Jeep 3.6 v6 turns 6400 rpm with higher compression than the XF and produces 290 hp all on 87 PON.

The question is whether the computer can retard the timing far enough for 87 PON but I have not seen any good reason for the 91 octane requirement.