The book "Inside the AS/400" has some really cool exposition on the unique aspects of the AS/400. If I remember right, the disks and memory are in a single address space, and of course, there is a relational database built in to the operating system.
I have seen the binary translation in action: it was required when POWER6 machines came out. On a POWER5 machine, I remember it taking around 20 minutes for an application that was around 20 MB zipped, but for software licensing reasons not all of the server models exposed the full performance of the processor, so that might not be representative. And at any rate, like the OP said, it's a one time thing, and then the translated version is kept.
I have seen the binary translation in action: it was required when POWER6 machines came out. On a POWER5 machine, I remember it taking around 20 minutes for an application that was around 20 MB zipped, but for software licensing reasons not all of the server models exposed the full performance of the processor, so that might not be representative. And at any rate, like the OP said, it's a one time thing, and then the translated version is kept.