I'm in the same zone and grow a lot of tropical hibiscus myself. I over winter mine in the greenhouse as well. What I do is cut them back 2/3 and root cuttings for 'insurance'. Every 3 years or so I not only cut them back but root prune as well so I don't have to keep upping the size of the pots they are in. They respond very well to this treatment and I haven't lost one yet.
They get their last feeding at the beginning of September and get cut down about the 1st week in October. I shoot for having the GH loaded with everything by the last week in October.
In the picture you can see them on the right.
