I would be extremely surprised that you would need 30 x86 boxes to reach the performance of a P8 box, on any type workload. By my experience with P5-P7 they can be faster for certain workloads then x86, but not that much.
You can't really compare the different chip revs apples-to-apples. P6 was a completely different chip architecture with much higher clock speeds that IBM abandoned because it didn't perform well. They make a lot of changes in each chip rev.
