More of an update to myself
I'm not quite sure how the advanced email routing functionality works. I added one google mail server and it delivers instantaneously (from a php mail form). When I added all other 5 servers as per google apps instructions, email took a really long time to relay (10 mins or so), so it seems the relays don't quite work the same as domain mx records (I had set different values under sort). Still curious as to how it hangs together and if I'm able to add additional smtp google mail server relays (for backup purposes).