I bought my 2004 LSV8 used with almost 14,000 on it. I now have about 56,000 miles. I have had warranty issues with tranny from as soon as I bought it but didn't realize it and a bad fuel pump at about 35K?, wheel bearing at about 50K? and 7 of 8 coils after idling on a 105+ day for 90 minutes due to freeway closure. All fixed under warranty and in my opinion not related to fuel octane issues.
I do several full throttle rolling starts with the LS every day in my commute. My commute is about 10 miles stop and go in the afternoons, about 15 miles street no traffic and then about 35 miles of frfeeway speed between say 50 and 80 mph.
Octane required depends a lot on driving style, weather, quality of gas, condition of engine etc. Everyone's experience will be slightly different.
I change octanes maybe once a month just to fool around and I take notes in my log book. If you run 87 octane and have a heavy foot you WILL have pinging from light to moderate. With 89 which is what I run probably 80% of the time, you might get a light ping on heavy acceleration on a hot day. I never have had a problem with 91 which I run maybe 10% of the time, and since it is in the manual it is the safe bet.
Interestingly I seem to get best mileage with 89. If I run premium and then change to 89 or even 87, the mileage goes up a point or two for one tankfull. Could be coincidence. A steady diet of 87 or 91 gets me about 1mpg less than 89 according to the info panel.
Pinging can be worse in older cars that are out of tune or have carbon deposits in the chamber. The can also affect gas mileage, so your problems may be related.
I get about 17-19mpg on my commute as described above. I have seen as high as 25 on a long trip LA to Vegas at 80mph.
Things I would check is air filter, plugs, O2 sensors, EGR, Plugs and maybe coils. Coils are a common problem with our cars, but usually you will notice a stumble under light acceleration at just below freeway speed. If they are really bad you will not miss the stumble and it happens at more speeds even idle.
Good Luck,
Jim Henderson