Be sure to put the SRX300 into switching mode as well: http://www.juniper.net/techpubs/en_US/junos15.1x49-d40/topics/concept/security-layer2-bridging-switching-overview.html
By default SRX300 is in transparent bridging mode, which sends all the L2 traffic through the network processor. By changing to switching mode it uses the L2 switch chip to forward L2 traffic at line-rate & only punts the L3 (IP) traffic up to the NP.
HtH.
-Blake