For anyone who's interested, I just consulted my Spanish girlfriend about the word "sudaca." To preface this it must be said that she's got a much different attitude toward racism from mine (I'm American). Since she left the rather traditional confines of Spain she's gathered a lot of friends of many races and is certainly no racist - but she still doesn't, for example, see why the Spanish basketball team's "Chinese eyes" picture from the Beijing Olympics is considered offensive.
So, her description of "sudaca" is basically this: it's definitely a pejorative word, and is considered relatively offensive, but it's not a racial slur on the order of the N word. She describes it as "a way to say South American in a condescending manner without actually speaking in a condescending manner." So if, for example, I were to say "Hey, you should meet my sudaca friend," it wouldn't be as bad as saying "you should meet my friend, who's a dirty South American c**t," but it would be along the lines of "you should meet my friend, who's South American and therefore on the inferior side of the Spanish-speaking coin."
So, long story short, it's only a mildly offensive word but lacks the potential affectionate use of "negrito." It also certainly breaks the FA's stated rule against using someone's nationality/ethnic origins in a disparaging manner.