The difference between 91 and 92 octane is probably less than 5° KR POTENTIAL, assuming your car is 'right on the edge' of KR to begin with. You had 19°, and there'* no way in hell you're going to kill that with an octane change.
Factory spec on fuel pressure is very loose. It'* listed as 41-47 PSI. However, you should have a MINIMUM of 44-45 PSI at idle with the vac line disconnected on the FPR to support that much of a pulley change. This is from my experience on both my S1 L67'*. Your fuel pressure should be a MINIMUM of 41 psi with the ignition on, motor off. After ignition off, it shouldn't drop more than 1 or 2 PSI, and it should hold that pressure for a few minutes at least, without bleeding down.
You'll likely find multiple causes for your KR. I would bet you have low fuel pressure, cracked exhaust manifolds, and carbon buildup all contributing. Go after them one at a time.