My MTU on pppoe0 is 1500 as my ISP supports RFC 4638.
I was using a default tcp iperf:
$ iperf -p 5002 -c 1.1.1.1
------------------------------------------------------------
Client connecting to 1.1.1.1, TCP port 5002
TCP window size: 18.4 KByte (default)
------------------------------------------------------------
[ 3] local 1.1.2.1 port 39371 connected with 1.1.1.1 port 5002
[ ID] Interval Transfer Bandwidth
[ 3] 0.0-10.0 sec 13.6 MBytes 11.4 Mbits/sec
I'll give a shot to mss-clamp.