This is not a NAT-T scenario.
NAT-T comes into picture only if there is a natting device in the path of VPN.
for example, say there is a vpn between firewall-1 and firewall-2
[Firewall-1]--------[router that does NAT/PT]--------[firewall-2]
Thanks for the reply. It turns out for some reason or another our NAT'ing router was not using all the possible ports. We removed that feature from the router and Cisco help us set up the NAT for a different interface on the FWSM for this particular traffic without affecting our other traffic leaving a different interface. Users reported good performance.