Best way to order health checks for 500+ sites

We have a similar setup where a handful of web servers host about a hundred sites. What we've done for what you describe as a "master check" is put an http monitor at the node level for each of these servers. So if any of the boxes fail the http check the node is marked down which then pulls it from all the pools it is in. The other advantage of this is that it's only a single http check against the server instead of the LTM checking it within each pool. Had that check been in the pool itself, the LTM would be checking the same thing once per pool (ie: hundreds of times instead of once).