Here's a helpful little write-up on the performance metrics of IBM's new P6-based System i 570 server, fresh off the production line as of this month.
While it notes that the P6 machine shows a one-third performance jump under the Commercial Processing Workload (CPW) test, the heavy I/O nature of that test is not likely to translate over to other workloads like the SPECjbb2005 benchmark.
Still, it must be noted that the Power6 box is rated at 76,900 CPWs, roughly a third better than the Power5+, even though the two boxes are priced about the same. In fact, to get the same performance out of the P5+ would require roughly $2 million in core upgrades and 5250 processing.
Another advantage is that you don't have to fully load the system with 15/OS. The lower-cost Application Server will do fine in areas where you don't need a database, and you'll have room left over for Linux and AIX loads as well.