said by OHSrob:
Caneris's ML-PPP implementation scales very nicely to 16 lines as well. They have one of the most reliable and versatile LNS I have ever had the pleasure of connecting to.
edit: I never tested an MTU of 1500 on a single line, it was always on 4+ lines.
But that is because you were using MLPPP. That is a different scenario. MLPPP allows for fragmentation and reassembly. Thus allowing for much much larger frames to be passed down the connection.
The point of RFC4638 is to both increase the MTU and stop the packet fragmentation.