Q&A: Will a smokebush survive in Northern Ontario?
A. You're quite right that smoke bushes are usually listed as zone 5 trees. However, my Sudbury (350 km north of you) smokebush has been very happy for about 10 years now. I lose some wood each winter, but by the end of June it is again more than a metre high and just lovely.
My advice to you is to find a protected micro-climate type location in your garden and watch it thrive!