The only Power based supercomputers I can think of are all from IBM. Does any other vendor use them?
Power CPUs tend to power either iSeries (latest iteration of AS/400 - System38 etc) systems or AIX based. The large AIX systems use lots of CPUs, cores and typically vPars - but I wouldn't call them supercomputers. Disk I/O is generally FC attached SAN, i.e. performance is achieved through off-loading. A typical SAN array contains gigabytes of caching memory, CPUs on each disk drive with yet more RAM and multiple optical FC links to each node.