There are many features that PowerPath has over NMP, the main one, as you said, boils down to performance and performance-tuning.
The below summary which was taken from Whitepaper "EMC PowerPath/VE for VMware vSphere Best Practices Planning" available from http://powerlink.emc.com
- Dynamic load balancing ¿ PowerPath is designed to use all paths at all times. PowerPath distributes I/O requests to a logical device across all available paths, rather than requiring a single path to bear the entire I/O burden. - Auto-restore of paths ¿ Periodic auto-restore reassigns logical devices when restoring paths from a failed state. Once restored, the paths automatically rebalance the I/O across all active channels. - Device prioritization ¿ Setting a high priority for a single or several devices improves their I/O performance at the expense of the remaining devices, while otherwise maintaining the best possible load balancing across all paths. This is especially useful when there are multiple VMs on a host with varying application performance and availability requirements. - Automated performance optimization ¿ PowerPath/VE automatically identifies the type of storage array and sets the highest performing optimization mode by default. For Symmetrix, the mode is SymmOpt (Symmetrix Optimized). For CLARiiON, the mode is CLAROpt (CLARiiON Optimized). For supported active/active third-party arrays, the mode is Adaptive. - Dynamic path failover and path recovery ¿ If a path fails, PowerPath/VE redistributes I/O traffic from that path to functioning paths. PowerPath/VE stops sending I/O to the failed path and checks for an active alternate path. If an active path is available, PowerPath/VE redirects I/O along that path. PowerPath/VE can compensate for multiple faults in the I/O channel (for example, HBAs, fiber-optic cables, a Fibre Channel switch, a storage array port). - EMC Celerra®, CLARiiON, Symmetrix, and non-EMC array support ¿ PowerPath/VE supports a wide variety of EMC and non-EMC arrays. Consult the EMC Support Matrix or E-Lab Navigator for a complete list. - Monitor/report I/O statistics ¿ While PowerPath/VE load balances I/O, it maintains statistics for all I/O for all paths. The administrator can view these statistics using rpowermt. - Automatic path testing ¿ PowerPath/VE periodically tests both live and dead paths. By testing live paths that may be idle, a failed path may be identified before an application attempts to pass I/O down it. By marking the path as failed before the application becomes aware of it, timeout and retry delays are reduced. By testing paths identified as failed, PowerPath/VE will automatically restore them to service when they pass the test. The I/O load will be automatically balanced across all active available paths.