I got the same subsystem id error when I was trying to flash my Sapphire 3870 with a new bios. And about those bios sizes, there seems to be ~61kb and ~65kb sizes for 3870. My originals were 61 but I force flashed with a 65kb and it worked. I am using Crossfire so it was easy to test on the slave card . Don't do it though if you don't have a backup plan on how to recover.