I guess it depends on your topology. If your 6509 switches are used as layer 3 switches using HSRP then even if only one 6509 is used as HSRP active for all VLANs and you have two IDSMs in there, you will miss all the traffic that is going through your HSRP standby chassis. For example, outbound traffic of a VLAN may be seen by HSRP Primary's IDSMs, but return traffic could be comming in both directions (HSRP Primary and Secondary 6509s). If you have one IDSM on each 6509s, then you are already using both of them. Please note that IDSM2's throughput is 600 Mbps.
We have configured the outside and inside Interface with official ipv6 adresses, set a default route on outside Interface to our router, we also have definied a rule , which also gets hits, to permit tcp from inside Interface to any6.
In Syslog I also se...