Ok im labbing this up at the moment in preparation for a new project, the basic layout is:
Firstly when I just setup a routed connection between router 1&2 everything worked fine:
Switch1 - Vlan1: ip igmp snooping
Router1 - internal interface: ip pim sparse-dense-mode
Router1 - external interface: ip pim sparse-dense-mode
Router2 - external interface: ip pim sparse-dense-mode
Router2 - internal interface: ip pim sparse-dense-mode
Switch2 - Vlan1: ip igmp snooping
Using PRTG I see that the bitrate all the way through is consistently ~150kb/s (my video streaming bitrate)
Then I tested with a DMVPN configured between Router 1&2 once again everything worked fine.
The only changes were, I removed ip pim sparse-dense-mode from the external router interfaces and put it on the tunnel interfaces.
When I use PRTG to observe the traffic, it was ~150kb/s all the way up to Router1 internal interface, then both of the DMVPN interfaces and Router2 internal interface showed ~300kb/s. The switch ports for client 1 & 2 only showed ~150kb/s.
So... what am I missing here? Why is the bandwidth double across the GRE IPSEC connection?