Does conduit terminated in backless gang boxes defeat the purpose of fireblocking?
I'm not a fire expert, but I doubt fire will actually spread through the conduit. Toxic fumes, sure, but not fire (unless the conduit is really short). Flames will, however, quickly jump through a hole in a 1 1/2" - 3" thick barrier.  

If you really want to take fireblocking to the extreme, you could seal the ends of the conduit around the cables with fireblocking material.