It's not about how fast you can serve them, but rather how fast they'll move through a line. people are bovine in nature, and frequently need to be prodded to keep moving.
I would suggest 3 lines, each with 2 servers. First server gets a plate and loads with beans, slaw, and a sandwich roll. Hands off to server #2 who then adds meat and passes plate back to customer. Repeat, quickly.
Server #2 may not seem to have a lot to do, but s/he's the one thanking the customer and interacting some, so their workload is more than it appears. Having #1 and #2 at arms length from each other will also help to keep things moving as it will discourage people from planting themselves in front of someone and just standing there.