Yeah, it works great on Linux. No performance loss at all, and you can connect to the host system from the emulated system, which pcap doesn't allow. On both systems, you need to build from clean with USE_NETWORK=1 on the make commandline. For Linux, the taputil.sh shell script sets up the connection for you - run it as root and then you can run MESS as non-root afterwards.
Specifically, it's something like this:
% sudo /bin/bash src/osd/sdl/taputil.sh -c username EMUADDR HOSTADDR MASK
where "username" is the user you will run MESS as, EMUADDR is the IP address to assign to the emulated machine, HOSTADDR is the IP address of your actual PC, and MASK is the netmask. On most modern Linux distros, you can click the Network Manager icon in the tray and see your host IP address and netmask. Also, any distro-supplied firewall needs to be disabled before running taputil or you probably won't be able to connect properly.
Run e.g. maciicx with -nbb enetnb and System 7.5 or later, enable Open Transport, and configure it as necessary. The IP address and netmask must match what you give to taputil, and the gateway and DNS server should be the same values as your host PC uses. Fetch 3.0 is a good "test" internet app for 68k machines.