For one, your numbers are off by a couple factors. 100Mbit=100mbps=12.5Mbytes/second & 1000Mbit/Gbit=1000mbps=125Mbytes/second.
If the most you've seen on a 100mbit network is 50% utilization, something's going on. You should be able to easily see 80-95%. Gigabit is a bit trickier, the...