Note that pppoe caches the MTU value of the parent device (em0 in this case) so if you increased the MTU of em0 after pppoe0 has been configured it probably didn't have an effect. You can tell this is what happened by ifconfig failing with an invalid argument error.
You can also check your kernel messages (dmesg) to see if there is an error like "No valid PPP-Max-Payload tag received in PADO". If you see this, then the ISP or telco's equipment probably doesn't support RFC 4638 and you can't do baby jumbos with pppoe. - todd
