Okay, after one month usage, I have to say the replacement SB2 has exactly the same battery drain whilst shutdown issue.
I've reported back the issue and advised to go through the same process as for the original device, which I rejected. I asked them to collect the device and do as much tests as they want in their lab and return a not faulty device.
Today, I received an email offering a replacement again and I responded that I don't want to go throught the same cycle unless Microsoft can gurantee that the next replacement device performs as per their specs. Unfortunately, I don't believe thay can. Otherwise, they should've already answered this thread by now.