Make sure your GUP has at least 20-30 versions of the defs, as well as your SEPM.
If Symantec makes certified defs every 4-5 hours, that means 4-5 downloads a day. The default setting is 3 to save in the SEPM.
If a machine is off for more than a day, the client machine will reach out to the SEPM for a full def package. That is over 100MB. Times that by your 17000 clients.
So, keep enough cached copies of your defs at the edge where it will make a HUGE difference for folks that have left their machines off over the weekend, and not kill the network upon start up.