87 octane?

I have a 94 Ski Nautique with a 5.8 HO engine. The TBI and the Protec ignition have been removed. A Holley Carb and a DUI distributor have been installed. The boat runs great so far as I can tell. I really don't run flat out. Pretty much nothing over 32 mph (with skiers). Usually 25-26 with foilers. I was checking out some other info in my manual and noticed that it specifies 89 octane gas. I have been running 87 octane for about 300 hours! The manual says you should try retarding the timing by 4 degrees if you run 87 octane. The DUI distributor instructions recommended an initial 12 degree advance and says you can try moving that forward until you hear pinging and then back off. I haven't advanced it over the initial setting. It seems the 12 degrees is a conservative setting with the DUI? I don't hear any pinging. If I had detonation would it really be audible?

While I am fine with spending a little more at the pump for 89 octane gas, I do ride on lakes where it's 87 only at the dock. Is this really a big deal? Should I retard my DUI setting? I checked my plugs and they look good and don't have any damage. I do try to take good care of my CC. I would appreciate a little advice. We ride once or twice a week all year round and I would like to keep it going.