I believe a hop back achieves similar results to dry hopping. Wort will have often been cooled at the point it goes through the hop back. Presumably as the hops get soaked they release more oils too.
I asked the wye valley brewery about their HPA recipe and they used a hop back. Their brewer advised my to dry hop (styrian holdings) rather than do a large, late addition. Presumably to manage bitterness and get as much floral notes from the styrian as possible.
I think the wort is still very hot when it passes through a hopback - straight from the kettle, through the hopback, then to the heat exchanger to be cooled. This may vary I guess, but the idea of passing cooled wort through the hopback doesn't make sense to me.