What is the benefit of upgrading Flight Management System to version UNS-1Ew?
Here is my understanding of the update. As others I believe your primary source should be your OEM.

Main purpose: Ability to use SBAS procedures and navigation

The purpose of moving from UNS-1E to UNS-1Ew is getting the SBAS capability. From the UNS-1Ew manual:


  The new “W” series of Universal Avionics Flight Management systems
  incorporate the Global Navigation Satellite System (GNSS) Space Based
  Augmentation System (SBAS) known in North America as the Wide Area
  Augmentation System (WAAS). Other SBAS systems in the world will also
  be incorporated.


SBAS details

SBAS means using WAAS, EGNOS and other GNSS augmentation systems. SBAS in short provides for correction of GNSS radio signal propagation errors in a limited area. This increases pseudo-ranging accuracy and positioning precision.

SBAS (space-based) is opposed to GBAS (ground-based) where correction broadcasting stations are on the ground, like the DGPS, a surveying GBAS with a positioning precision up to 10 cm.

EGNOS measuring network (SBAS for Europe area):


Source: Wikipedia



Additional benefits of the update

From UASC documentation:


  What else you get with an SBAS-FMS upgrade:
  
  •  Performance-Based Navigation (PBN) including Precision-Area
  Navigation (P-RNAV) compliance and Required Navigation
  Performance/Area Navigation (RNP/RNAV) up to 0.3 NM approach accuracy
  
  • WAAS LPV / EGNOS APV, LNAV approaches 
  
  • Mode S Enhanced Surveillance support
  
  •  Accurate SBAS-GPS information provided to the onboard Terrain
  Awareness and Warning System/Enhanced Ground Proximity Warning System
  (TAWS/EGPWS) and Traffic Collision Avoidance System (TCAS)
  
  •  Large database capacity for worldwide navigation databases


You can read "The Benefits of SBAS" on UASC site.

And if you need more information the UNS-1Ew manual is also available.