We are testing MT x86 for PPPoE termination now. One box has ~800 sessions/~120Mbit/~20K pps with 80% CPU load. Processor is Xeon 2.8 (SMP off because of 3.12 bug, but not a big difference between 3.11 with SMP turned on). MT support told me that in 3.12 they should have simple queue optimizations to lower CPU load, I did not notice. So far customers on this test MT box did not complain about slowness, packet loss, etc - MT runs fine.
In comparison to MT, we use plain linux PPPoE termination boxes with 2000 sessions/300Mbit/80K pps with CPU load about 60%. MT pluses are configuration simplicity, gui and other bells & whistles. Plain linux advantage is performance. Connection tracking is off in both cases.