In baseball, is it proper to pluralize "RBI"?

In baseball, an RBI is a Run(s) batted in. So it's already "plural".

So if I were to say, Paul Konerko had five RBIs. Is that correct, or should I say Paul Konerko had five RBI.

Solution 1:

Yes, and it's spelled without the periods, i.e. RBIs.

See Wikipedia's List of Major League Baseball Players with 1000 RBIs, for example.

Note, however, that when you say the whole words, you pluralize runs, as in runs batted in.

Solution 2:

RBI is an acronym, thus a noun: singular RBI, plural RBIs. If you want to say the words, say them, don't use the acronym. You would not use the plural for 'an RBI double' to say 'runs batted in double' meaning one run. Confusion emanates from ending the term with an adverb, an awkward construction. If the phrase had been coined 'batted in run', BIR, instead, with the noun at the end, there would not be this confusion. Also, decimals are not used in acronyms.