When I was bored yesterday I dealed with HDD decryption again and looked through the game folders on the decrypted HDD. The Def Jam: Icon USA Demo seemed interesting as it only works on US consoles. While looking for the answer I noticed a folder dj3-ps3-opt in /USRDIR/spu/. In there are several files like elf_sputhreads_apply.bin. I opened some files in a simple HEX editor and noticed that the files are not encrypted.
Aswell it looks like a binary. It does not have the default binary header used by SPU .self files compiled with the SONY PS3 SDK. It may be compiled with a third party compiler or with the IBM Cell SDK. I did not check that, yet. Well, come back to check for updates regarding the research!