Well put. I didn't want to get too technical. ;-)87 octane fuel will cause detonation under conditions where 91/93 will not so its use can reduce power. This doesn’t happen only at WOT but can happen any time you quickly demand more power at low rpm, even at part throttle. This means during passing, pulling away from a light, etc.
I’ve run back-to-back tanks of 87 and 91 twice in my turbo CE to compare, and I can feel the difference during normal (not WOT) driving at rpm well under 4000 rpm.
And yeah, I too definitely feel the difference (particular torque response) with on-demand load, even below 4000RPM.