The upgrades in most of these miners is purely firmware and Bitmain's comfort at running higher freqs. Think about it, the ASIC chips are not changing--they're hard coded and they aren't about to re-spin the fab. Bitmain did do a slight form factor change on the D3, but that was inconsequential. From playing with the settings, it's easy to add 10-15% efficiency to a miner as long as it's properly cooled. Some even benefit (or at least perform equally) from undervolting.

Just one guy's advice, but I've increased my profits by playing with my Advanced page settings to get more out of the chips without adding discards or overheating.