Multiple IPSec tunnels to the same remote peer

To tunnel multiple subnets over IPsec, typically you'd just add these "routes" (SPIs) in your phase-2 settings on both ends of the tunnel. IPsec doesn't really have a concept of "multiple tunnels", because ESP traffic is connectionless and traffic traveling to the peer should be routed toward the one that will accept it.

How are you able to determine that there is a 1 gig bottleneck in your setup?

Have you tried doing this with the BIG-IP and the peer device connected on a network that you can locally control?