Plastron shrouds on the A340 underwing are there solely to cover fuel pump wiring from the main and standby fuel pumps. This means no wiring from the fuel pumps goes into the tanks. There are two shrouds on each wing, inboard of the inner engines.
The speed of the A340 was chosen from surveys with the airlines when the original airframe proposals were put forward. The theory being that the Mach .82 versus Mach .86 (approximately 20kts faster, or around 20 mins for a ten hour flight), on long sectors did not warrant the extra fuel consumption, The -500/-600 cruise slightly faster at Mach .83, so I guess Airbus changed their minds!
A330 cruise is also around the Mach .80-Mach .82 which is commensurate or slightly faster than a 767. The 777 is a much faster machine than the 340/330, as is the 747.
All down to a compromise between fuel burn and load versus range.