Hey Mike,
I wouldn't be so sure - I think you'd first want to actually try the new 1Gbps link on your MX100 before making a decision.
Our sizing guides were traditionally pretty understated, and we will actually be releasing a new one shortly with updated numbers that breaks down expected throughput for different traffic mixes with different firmware.
If I'm looking at real world tests for MX 13 firmware, an MX100 shows as follows (if you're staying within expected number of clients):
Full security enabled (AMP/IPS/Traffic Shaping/Port Forwarding/CF Full/other services): 790Mbps - 950Mbps
Basic Security(AMP/IPS/Content Filtering Top-Sites): 795Mbps - 955Mbps
Basic (NAT/DHCP/L3&L7FW/AutoVPN): 850Mbps - 975Mbps
Watch for this new sizing guide that will have additional details.