A colleague found an interesting piece of information on page 23 in the "Release Notes of AIX 6.1":
AIX Version 6.1 with Technology Level 6100-04 supports a maximum of 64 processor cores and a maximum of 128 logical processors. The logical processors correspond to the total number of Simultaneous Multithreading (SMT) threads. This means that the maximum supported configuration is 64-core POWER5 or POWER6 in SMT2 mode (2 threads per core).
Looks to be the reason why there are just a few published benchmark for a larger machine. Of course you could use LPARS, but then the SAP-SD benchmark would be a 3-tier result and not a 2-tier one fore example. And as far as others and i've read through all the service pack docs, this limit is still in AIX 6.1. Given this limit, the maximum single image Power7 configuration would be 8 Power7 chips when used in SMT2 and 4 chips when used with SMT4.

PS: The scalability of Solaris? Just look here.