I don't know why the 8553 PT is so cheap but I thought that maybe it wasn't as durable as the 1008 or the 521 SD. I'm wondering (and may never know) why the engine barfed the head gaskets given that it seemed to have a reasonable compression ratio.
This is an approximation of the previous setup:
Like someone mentioned, it may have been the tune. It may have been the torque sequence during assembly, it may be an uneven deck surface.
It may have been 87 octane in a car that sat for 4 months.