NATing the Office Mode address isn't really a matter of increasing security, but ensuring proper routing behavior for traffic returning to the firewall for encryption back into the Remote Access VPN tunnel. Normally you don't need to NAT Office Mode addresses, and you just specify an IP subnet for the Office Mode assignments that your internal network is guaranteed to return to that same firewall in a symmetric fashion.
One situation where you might NAT Office Mode addresses is in a Multiple Entry Point (MEP) VPN scenario which involves several firewalls at different locations with their own Internet connections, and a backend WAN connecting all the different sites as well. In some cases Remote Access VPN traffic might enter the inside network via Firewall A, but then try to asymmetrically return to the Internet through Firewall B located at another site which will not work. If you are not able to correct the routing inside your network due to technical or political reasons, performing NAT on the source IP of the Office Mode traffic at Firewall A and making it an address that is guaranteed to be returned to Firewall A symmetrically can help avoid this issue.
--
Second Edition of my "Max Power" Firewall Book
Now Available at http://www.maxpowerfirewalls.com
Gaia 4.18 (R82) Immersion Tips, Tricks, & Best Practices
Self-Guided Video Series Coming Soon