I am wondering if there is a way to query all active interfaces on a switch for either rx/tx load? If so, how would you go about implementing that so if any interface rx or tx load is hitting 90% to email the admin? Also, not sure what the consequences are of event manager monitoring all of the intefaces on say a 6509 w/ sup 720 and the remaining modules filled with WS-X6748-GE-TX? In other words, is EEM resource intensive?
Have a look at https://supportforums.cisco.com/thread/2245225?tstart=0 as a primer for grabbing all interfaces. For the Sup720, you'll need to follow my instructions for converting to Tcl. Once you have the list of interfaces, you can pull the load from "show interface" and alert if the ratio is over your threshold.
EEM is a little bit less resource intensive than if you were to manually perform the steps in your policy. So it will depend on how often the policy runs as to the full impact it will have.