What's the best Veeam design to maximize IPsec throughput?

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I have established an IPsec tunnel from Verizon Gigabit Fios on the East Coast to Centurylink Gigabit fiber in the inter-mountain West for the purpose of establishing Veeam Replication.

Our Veeam setup includes three Proxies at the source and three proxies at the destination, each with 8 CPU/Cores and 8 Simultaneous Disks/VMs, all W2K8. The Veeam Server is at the destination in the inter-mountain west and counts as one of the proxies. Latest patch for Veeam 7. There are about 24 large VMs being replicated at least once hourly.

The problem: I can rarely achieve cumulative throughput of greater than 80 Mbps with Veeam, and when all jobs kick-off simultaneously, the bottleneck is clearly network.

I have recently been trying to analyze throughput limitations with iperf, and been somewhat mystified by the results. When I run:
iperf -c 192.168.100.29 -P 10 -t 60 I might get: [SUM] 0.0-63.4 sec 89.2 MBytes 55.2 Mbits/sec
iperf -c 192.168.100.29 -P 40 -t 60 I might get: [SUM] 0.0-63.4 sec 382 MBytes 101 Mbits/sec

I had previously thought those were pretty good results for any VPN, but then I thought to set-up three clients and three servers and was surprised to see the VPN gracefully handle all of the traffic! Running three of each of the previous tests simultaneously, I saw almost identical results. In other words: the VPN was capable of pushing 300 Mbps.

The question: If the VPN can push 300 Mbps (or more) across the WAN, why does a single connection from one IP (at the source) to one IP (at the destination) seem to be limited to 60-100 Mbps? How can I tune Veeam to take advantage of greater network/VPN availability which, apparently, exists?

THX,
-J