The short answer is yes, they are pronounced the same. But "Boss' car" is not the correct use of the apostrophe. To show possession in the singular, you add apostrophe + s. So your first example is correct: "my boss's car." To show possession in the plural when a word ends with s, you add the apostrophe at the end. But the plural of "boss" is "bosses," so it would be "the bosses' car." "Boss's car" and "bosses' car" sound exactly the same.

Boss' car is wrong. The possessive form of the singular noun boss is written boss's. The plural of boss is bosses. The possessive form of bosses is written bosses'. The words boss's, bosses, and bosses' have the same pronunciation, which you have written as "bossiz".