Say if I got the HG612 how would I know it's on the latest firmware without unlocking it?
You cant - until you have an unlocked version you cant access any of the details.
People say the ECI /r syncs slightly faster than the HG612 but what about the Hg612 with the latest firmware is the ECI /r still faster?
I'm not sure about that, and it certainly doesnt appear to be the experience of the majority of users - certainly those on this forum where the BCM based chipsets appear to have the upper hand. Im on an ECI cab, yet BCM chipsets give me anything from 3.5Mbps - 7Mbps more.
The Lantiq chipsets do sometimes appear to come into their own on some of the longer or noisy lines perhaps in a similar way to how the old 2Wire 2700's used to be very good on very long lines, but rubbish on short lines.
Its the same with the BCM chipsets - most see an improvement with the BCM63168 over the BCM6368, but there are a couple who report it generates more CRCs.
At the end of the day you may just have to see what works best on your own individual line and how noisy it is, but I'd say that the BCMs tend to do better than the Lantiqs in the vast majority of cases.