Essentially, are we arguing that Seneca's use of "Summum Bonum" (or "highest good") as opposed to another phrase, perhaps the available "Maximum bonum" (or "greatest good") is an indication that Seneca misunderstood a nuanced, yet crucial distinction between "high" and "great"? What leads us to believe that Epicurus recognized such a distinction?
I personally think "Summum" might be a better rendering than "Maximum": the ancient Greek word Epicurus employed to describe the fullness of pure pleasure in KD4 was AKPON, meaning "extreme", "acutest", "intense", "sharpest", "end", with the added connotation of "peak", "highest", and "mountain top". He chose to describe the limit of pleasure, not only in terms of a general magnitude, but, specifically, within the figurative context of "altitude" (i.e. "highest").
That being said, Epicurus is not specific to a single term. There are multiple words in ancient Greek he employed that describe a "magnitude" of pleasure. He uses MEΓEΘOYΣ (KD3) meaning “great”, “loudness”, “quantitative limit”, “maximum”, “upper limit”, “total power”, the “full measurement of greatness”, ΠOΛΛAΣ (KD4) meaning “much”, “many”, “often”, “might”, “great”, “strong”, EIΛIKPINEΣTATH (KD14) meaning "great", "abundant", "bountiful", KYPIOTATA (KD16) meaning “essential”, “principal”, “dominant, “most important”, “primary”, as in the Kuriai, MEΓIΣTA (KD16) meaning "greatest", ΠΛEIΣTHΣ (KD17) meaning "most", "greatest", "largest" ... that's just a quick sample of the magnitude-expressing words Epicurus uses. Given this, is there really that big of a different between "Summum" and "Maximum"?
If Seneca used "Maximo Bonum" (or "loudest pleasure") it could still carry the same meaning of 100%. Epicurus was willing to describe good, full pleasure with MEΓEΘOYΣ, which can connotate the magnitude of audible volume, instead of AKPON, the magnitude of altitude, as well as the general concept of the greatest measurement of a thing.
OR, is it our suggestion that any adjective implying "greatest" is inappropriate to link to the noun meaning "good"?