I didn't believe that security flow would not apply on SRX ports in ethernet mode so I ran that same test in my own lab and was surprised to see you cannot apply security policies to switched traffic and there is no flow processing for local switching.
I'm suprised because the documentation sure implies to me that we can secure mixed layer 2 and 3 traffic on the SRX but that is not the case. Only the layer 3 traffic hits the flow processor in mixed mode.
So clearly there won't be as large a hit to move switching to the SRX as I thought. But I still stand by that the SRX will not have the same full line rate availabe on a modern switch. We would need a full line rate test to verify what the capacity actually is.