Lower octane fuel (down to 87) was also an owner's manual option in
earlier C4s.  The Electronic Spark Control (ESC) was supposed to handle
the knock problem (sorta).  I've tried both 87 and 89 octane in my '89
and don't like what I'd call "combustion roughness" (crappy timing) even
at part throttle.  I've used 87 octane in an '98 on long haul trips and
found that the C5 adjusted much better (superior head design probably
helped.)  Dad has extolled the virtues of the C6 with reduced octane
fuel.  So, engines and fuels do get better as we forge ahead.

My LT1 & LT4 driving experience is very limited so I can't relate any
experience with fuel in those cars.  Both had better combustion chambers
than did the aluminum headed L98s.  Maybe someone has found that they do
better with 87 or 89 octane than did the L98s.  Just looking at the
engine specs, I'd feel 'less uncomfortable' screwing around with 89
octane in an LT1 than in an LT4.

Hib Halverson has a discussion of fuel quality, octane, flame front
travel and heads at << http://www.idavette.net/hib/fuel/index.htm >>.
Although targeted at racing fuel, his discussion makes practical street
sense to me.  After Katrina, GW relaxed the wintertime environmental
rules on West Coast gas.  Calif refineries decreased the amount of oxy
compounds in fuel and my C4 started sounding smoother and feeling
quicker off the line.  I didn't get the C5 until well after the Katrina
changes so don't have personal experience with the LS1 engine and the
new/old West Coast gas formulations.  (And, I've only topped up the tank
three times on this C5.)