In the current NAS market, it is downright impossible to talk of ARM and high performance together. The most powerful ARM-based NAS units have been based on Marvell's ARMADA processors. They usually come with dual gigabit network links and typically target the SOHO and low-end SMB market. Intel's offerings have had a virtual monopoly in the other tiers of the market. Synology is set to change all that with their latest offering - the DS2015xs with native 10G capabilities.
The SoC at the heart of the DS2015xs is the AL-514 from Annapurna Labs, an Israeli startup that is still in stealth mode. The company has declined to speak to the media as of now. However, tracing some coverage of Israeli VC firms reveals that Annapurna Labs was founded in 2011 with the intent of bringing ARM-based communication processors to the market. Datasheets of SoCs from Annapurna Labs are not currently available to the public, but Synology was kind enough to divulge the following details (which, I suspect, can be gleaned via SSH access to the DS2015xs):
From the link below you can find more information:
AnandTech | Synology's DS2015xs brings ARM to High-Performance NAS Units