Yes, "SAKO" is an acronym (a word made from initials). Acronyms tend to be pronounced under the rules of pronunciation of whatever language is being spoken. In most languages "sako" comes out with a soft "a" as "sahko" (or nearly the "sock-o" that has been described). However, in English the pronunciation rules make it a long "a" or "say-ko". So if it were purely an acronym then its pronunciation would be "acceptable" either way, depending on what language you are speaking. (However, I don't think anyone would accept the somewhat Cockney "sack-o" as anything other than grating to the ears.)
The rub is that "Sako" has long-since become a proper noun, not just an acronym. Proper nouns are pronounced appropriately as they are spoken by those to whom the name applies. Just as we wouldn't expect a speaker of various European languages to call the National Aeronautics and Space Administration "nahsa" while we call it "na-sah" with a short first "a" and a soft second "a", European speakers expect to hear "sahko" when the name "Sako" is spoken.
Regardless, I've never walked away from a good deal on either a "Sayko" or a "Sahko" and have no criticism of anyone who uses either pronunciation.