"Pi's" contains a grocer's apostrophe. Bad, bad, bad. Hippy is quite correct when he says it indicates the possessive.
As to the comment about "2 Pi R", that is a mathematical equation, not a form of prose, so the rule that is being implied is not applicable. Alas, Archimedes was Greek, so perhaps we shouldn't look to him as a scholar of the English language. I doubt he ever spoke, or heard it spoken during his lifetime.
As far as I am aware (and I'm not a grammar expert but I am sensitive to glaring errors like grocer's apostrophes) it is grammatically correct to say "Raspberry Pis", even if it does look inelegant. Alternatively, you can avoid this minefield altogether by saying "Raspberry Pi computers" instead.