108 0ctane
Very funny. Now, what is the octane rating on the pump at your gas station? Typical numbers are 87 for "regular", 89 for "mid grade", and 91 to 93 for "premium". Your Harley is designed to run satisfactorily on 91 octane (R+M/2). If it doesn't run properly on the fuel grade Harley specifies, then you need to complain to H-D. It sounds like your dealership is just trying to sell you a $500 SEPST rather than do some diagnostics to see if perhaps you really do have something wrong. And persistent detonation is not "normal" or acceptable, nor is it the fault of the EPA. Last time I checked, the EPA does not build or sell motorcycles. Also last time I checked, lots of other manufacturer's who meet the same standards (motorcycles) or much tougher standards (passenger cars) don't have persistent detonation problems.
Yes, if you have the bike tuned by a competent individual you will probably be much happier with the way the bike runs versus the stock calibration. That is always true. But it should not be necessary to have a stock bike "tuned" to eliminate severe detonation. Call H-D, then find a better dealership.
Jerry