Your doing the right thing, but at the wrong time,
If in october time you really take it back, it wont grow as much as the winter is just around the corner and growth slows right down,
By doing this at the end of the growing season you'll help the wisteria by not having strong winds and snow do damage because there wont be a mass of wisteria to get a hold of,
Now come spring the roots will put growth back but if you reduced the height and thick growth then the power of the roots growth will go into what you've left and the flowers will still come, once the first flowers have died away (here in france its now june/july time) give the wisteria a early tidy up clipping but not to much, and you'll get a second flowering in september time and this now brings you round to your winter harder clipping in oct or when the flowers have died.
You have now come full circle ref the clipping of your wisteria.

A perfect answer, Dungy.
May I just add that builders once chopped our huge wisteria almost to the bone one March (much against my wishes, but scaffolding had to go up) It didn't produce much that year, but the following year broke all records. So, treat 'em mean - they love it.

Thanks. The trouble is, I can't leave it until October before cutting. The plant would swamp the house. My concern is that needing to cut back some seven times after flowering, I reduce the number of flowers for the following year. But it's reassuring to know that you can prune them hard.