Help me figure out this J Stars STPZ compression

[Dakilla]

I've been looking on how I could uncompress these .pak files for a long, long time now...
I looked into many forum threads trying to figure out how to do it. While almost all of them have relevant information, none is actually 100% on point and leaves something to be worked upon.

Here are some of the threads I read:
http://zenhax.com/viewtopic.php?t=831
viewtopic.php?f=10&t=11342&hilit=stpz
viewtopic.php?f=16&t=9129&hilit=stpz

"Why am I starting a new thread about it then?" you might wonder. Well, it's simple, yesterday I was browsing the
PS3 memory to get any clues and I found out the offsets for the uncompressed versions of the files...
So I'm attaching a file example of both the compressed and uncompressed versions with hopes that someone can figure out the compression algorithm that I cannot for the sake of me find out :S

Take into consideration that my goal isn't uncompressing and unpacking the files, but yet uncompress, edit and then compress it back!

Note: The uncompressed file is a memory dump from the idstring (STPK) of the file, until the offset of the next file's idstring(STPK)... so, the filesize is likely correct but may be a few bytes off, i'm not sure