That's all we can do is think and assume as to what these 300 bytes actually are. Until somebody who is familiar with eboots (especially the elf inside) posts, there is not a lot more we can do.
Like I said on my original post, it is just an observation.
Also, the 1280 is repeated again at the "section_info_offset" of 290 which should also be 980. I am assuming that any reference to the elf header length has been changed to 1280 to compensate for this extra 300 bytes.
That could also mean any offsets pointing to areas AFTER this 300 byte area would have to be changed accordingly. Things like the Program Sections etc. as they would have moved further down the file by 300 bytes.