Would J1900 be capable of handing Sophos UTM?

Had similar setup - but with a Gigabyte-Board and smaller SSD.

With all Security Options on - I was never near on any performance issues at all.

Have a look at this guy's page: http://networkguy.de/?p=269

Build my Home UTM on the J1900 recommendation there.

Worked like a charm.

How many users and what is the wan speed?

I've just a small Lab-Farm of 6-10 Servers + several Home-Use-Devices and two Users with Client and Smartphone behind it.

I do not reach the IP-Limit of 50 IP's at all.

But it's fast enough for a permanent VPN-Tunel to my parents place, Netflix-Streaming, Surfing and Working.

My Limit ist my ISP-Upload-Speed...