The Roman and Persian empires

[Furious Mental]

Persian satraps were practically independent kings, depending on how far from the centre of power they were. Roman provincials governors generally weren't. But in any case a "confederacy" implies that a group of states have voluntarily decided to be governed by a small national government with limited powers. Frankly the Roman empire was the exact opposite- nations were conquered and forced to accept the rule of a very large and overbearing imperial government.

"if the Roman empire had been a significantly closer copy of the Persian empire, what differences would you expect to see?"

Well that is like asking "what if up were down". There were huge gulfs between Roman and Persian society, and many of the societies they conquered were different, so the structure of the empires was different. For instance, the method of government in the provinces, at least up until the foundation of the principate, was inextricably linked with the republican system of government as the proconsuls and propraetors who governed the provinces were those who had just finished their one year term as consul or praetor in Rome, and generally they were limited to a one year governorship (longer appointments needed special legislative authorisation). Eventually Rome became a monarchy in the proper sense of the term anyway, i.e. more like Persia.