I have a typical firewall up (router) that is difficult to punch a hole in.

If you're talking about a hardware router, you should be able to port forward from the router's internet facing interface to an internal IP address that's hosting your I2P/I2P+ router. You only need to do that for the public I2P port, for both TCP and UDP. You'll find your router operates much, much better if you can overcome the firewalling of your I2P port. The assigned port is indicated on: http://127.0.0.1:7657/confignet

Regarding general activity, I2P+ is much more aggressive in blocking, banning or otherwise ignoring routers on the network that may be participating in attacks or are poorly configured and maintained, so that's probably why you're seeing a difference in network traffic. Keep an eye on the banned peers counter and the tunnel build success percentage in the sidebar in I2P+, these will give you some idea of the network health.