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When I had SC I worked with some Aussies who couldn't get SC as they hadn't been here 3+ years - but they got BC fine (or at least in the time it took for the wonks to process it).
That's because SC (and DV) is a proper (point in time) security check.
BC says that you are who you say you are, but nothing more than that - even the DVA say that it's not a security check. And you'd still need higher these days to work anywhere that is a terrorist target.
Mine took 5 days, and yes, it's client specific. So if you swap contracts then you need BC all over again. It's a PITA if (like me) you've had lots of short term contracts in the last 5 years.