Wow, interesting. I would never think to buy some dedicated product to do something like this. I would find an old box somewhere, install OpenBSD, and resume my day.

I have a Soekris net5501 that I use as a router and it does 100MBps between two of the NICs without much trouble (with pf). I have a Gigabit NIC in there and it manages to push around 300-400MBps between it and another box on the network (with an admittedly shitty NIC that craps out on any frame bigger than 1500 bytes).

The gigabit NIC uses more power than the box itself. A whole x86 machine with 5 NICs running the latest version of a modern OS that uses less than 20W of power. And of course no fan or other moving parts.

Anyway, copying packets from one network to another is not a particularly difficult task. "pass out keep state" is similarly simple. So really, this is a task that doesn't require a "vendor", it simply requires a computer that has more than 32M of RAM that can talk to NICs.