Without encryption you can get a bit more than that on ALIX, around 80Mbit/s if you're lucky.
With its RAM you'll be limited by the number of simultaneous connections (states), by default it will only allow ~21,000 concurrent states which would roughly translate to ~10,500 end-user connections (one state entering the firewall, one state leaving). That can be nudged up a bit, but that box is tight on RAM already.
How many users you can support depends on how many connections each of them will make. Light users may only need a couple dozen states each, heavy users and torrenters may need thousands.