Not really ...
You havent't really understood how APEX works.
HDS Server Priority Manager will only limit IOs of non prioritized servers on the USP / VSP ports. That's all.
APEX enables you to define service level objectives and allows setting QoS levels like max latency in ms and min IOPS or min MB/s for certain servers. You can define up to 16 priority values based on bandwidth and latency.
The IOs are controlled in the server IO driver stack rather than on the XP / P9500 port.
Let's say an application A has been definde to achive a max IO latency of 10ms (measured at the server level). The other servers and applications do only get restricted if it is required to achieve the <10ms for App A.
Other important and unique solutions HP adds to the P9500 are the metropolitan clustering solutions HP Metrocluster for Serviceguard on HP-UX and HP Cluster Extension for MSCS on Windows, RedHat Cluster and Suse Cluster on Linux, HACMP on AIX and VCS on Solaris.