I've always understood it to be 552 (units of) horsepower, where units of is understood and rarely spoken. But Merriam-Webster and Wiktionary both list horsepower as the actual plural form.
Either way, horsepowers is definitely not standard.
It seems that the current consensus is “don’t change” (-ys).
Swan 2005 cautiously says that "proper nouns usually [emphasis mine - Alex B.] have ys".
the Kennedys (not the Kennedies)
There’s a punk band, Dead Kennedys http://www.deadkennedys.com/; there’s also aTV show, The Kennedys.
the Willoughbys
the Wolfs (not the Wolves)
the Henrys
the two Germanys
Februarys
Marys (not Maries)
Huddleston and Pullum 2002 give the following rule for proper nouns: “the base always remains unchanged in both speech and writing” (p. 1595).
I was able to find one reference only (Chalker 1992 in The Oxford companion to the English language) that argues for the form two Maries.
The majority of grammars argue for “Marys”:
the three Marys (The Oxford guide to English usage, p. 40)
the little Marys (A comprehensive grammar of the English language, 1985)
three Hail Marys (The Cambridge guide to English usage, 2004).
Google “both Marys”, “both Maries”, “two Marys”, and “two Maries”. By all means, the unchanged form (Marys) is much more common.
A note on the Rocky Mountains (Rockies). Its derivation is not a Rocky =>the Rockies.
Re: Tony Awards (Tonys or ?Tonies). Its official website uses "Tonys."
The variant "Typhoid Marys" is also more common than "Typhoid Maries."
Best Answer
http://grammarist.com/style/last-names/
Plural last names
Making a last name plural should never involve an apostrophe. The members of the Johnson and Smith families, for instance, are the Johnsons and the Smiths, not the Johnson’s and the Smith’s.
Last names ending in s are no different. Most nouns ending in s are pluralized by adding es. This applies to last names as well. The members of the Edwards and Doss families are the Edwardses and the Dosses, not the Edwards’s and the Doss’s. Your spell check might disapprove of the correct forms, but spell check is wrong on this matter. If you understandably find words like Edwardses a little too awkward, consider rewording to avoid the plural. For instance, the Edwardses can become the Edwards family or the Edwards household.