spark knock IS a Harley thing.

HD has tuned your engine extremely lean and with maximum timing advance.

I completely disagree with your shop opinion that it is ok. Spark knock, otherwise known as detonation or pre-ignition will destroy your engine.

this is what it does to pistons:

this is what it does to valves:

octane boost will ease the symptoms, but that is not the cure.

a well tuned Electra Glide gets 35-45 mpg. my guess is, you are doing a little better than that.

A HD stage 1 download will help. it richens the fuel mixture, tweaks the timing and raises your rev limit to 6200. the dealership can tell you if it is installed or not by hooking their laptop up to the bike.

if that doesn't eliminate the spark knock, you will have to add a tuner. HD has a few download tuners which make your ECM adjustable. Power Commander is a add on tuner from Dyno Jet that also allows you to richen up the fuel map and tweak the timing map.

with the power commander, you can retard the timing 4 degrees over the entire timing map and add a percent or two to the entire fuel map, in steps, until you have eliminated the spark knock.

I hope that answers your question and if not, please ask for more detail or any clarification you might need.

Steve