Hi joddle,
I think the problem may lie with your router.
The 100 = 100Mb/s ( Megabits / sec )
100Mb/s = 12.5 MB/s (Megabytes / sec )
[ 8 bits in a Byte ]
So - no matter how fast your NIC cards are - your transfer speed is limited by the speed of your router.
I hope this makes sense ?