APM as a Captive Portal

We've been using APM to prompt for authentication before users are permitted to access the internet by integrating the F5 device with some of our other network infrastructure. So far we've only been trialling this on our smaller sites with no issue (no more than probably 50 users I'm guessing).

 

I'm a little concerned about excessive load on the system if we applied this solution to the entire organisation. Firefox now polls constantly to detect captive portals, every few seconds. Plenty of other applications, Windows and mobile devices also do it (though nothing near as tenaciously as Firefox it seems). Is APM up to this task considering how many pointless sessions are going to be created and then abandoned?

 

Any advice on this, any gotchas to watch out for?