NTFS formatted USB on UEFI - bootable on which models?

To our surprise and delight we see that the E7x40 laptops are able to boot our NTFS formatted image deployment pendrives even in UEFI/Secure Boot mode - this seems contrary to the UEFI specs FAT32 requirement, and it definitely does not work on our E6x30s. Does anyone have a complete list of what models (and required firmware levels, if any) are capable of this particular trick? It's a great convenience not having to split up our WIM file to adhere to the <4GB limitation of FAT32.