I've set up an accel-ppp server here and I'm slowly migrating the pppoe clients from mikrotik to it. I also configured an accel-ppp ipoe for testing. :D
i believe btw more is possible, but in Lebanon electricity is switching on/off during the day, so i have "surges" at least 4 times per day when majority of users login/logoff at same time
sometimes i am thinking about hardware offloading, we even discussed with guys here, but till now all gear that can push in hardware (FPGA) 10Gbit is more expensive than putting few more servers with X520