Which is the difference between firmware Developer (*.**.1540.*) and (*.**.401.*)?
The x.xx.1540.x is for Unlocked / Developer Edition with CID BS_US001 / BS_US002 - America.
The x.xx.401.x is for Unlocked WWE ( World Wide English ) CID HTC__032 or HTC__001 , etc. - Europe.
It seams that the updates will be released for DevEd first , after that the rest of the world.
Sorry to take this thread slightly off-topic, but I just do't get when/where this misconception started. WWE is not some hardware flag, there is no WWE variant of the M8. WWE is a (overused in my opinion) software flag. A large amount of HTC devices, including region and carrier specific, have this flag. HTC Asia ROMs have WWE, carrier branded have WWE, US Unlocked and Developer Editions are WWE, and also European Unlocked are WWE. As I said, it's just an overused software flag.
The difference between 1540 and 401 firmwares are that 1540 is US Unlocked / Developer Editions and 401 is European Unlocked Edition. For a specific phone to be a WWE as you call it, it would actually have to be available world-wide and not one specific region.
I've had my phone since near the initial release, and HTC seems to alternate between who gets updates first - 401 and 1540. Either way, these two get updates WAY faster than any other variant.
As for the other poster, there really isn't much difference between the two. The Developer Edition has some scary text in it about how your device still belongs to HTC, but it is functionally identical to US unlocked and WWE.
The text you refer to is when you S-OFF. Any device with S-OFF wil. have that text. The difference with Developer Edition and US Unlocked and other unlocked is that it ships with the bootloader unlocked. Which means no going to HTC no having your data wiped and no worrying about relocking the bootloader for OTAs if you stay S-ON.