I was clocked doing 184 km/h on the same stretch of highway (S/B 404 somewhere north of Hwy. 7) back in November. I did a lot of research and interviewed several lawyers (retained two of them at first before deciding which one I wanted to go with), and it paid off in that I managed to keep my licence despite the severity of the offence. My lawyer managed to convince the prosecutor that there were exceptional circumstances in my case (of which I had proof and which the officer's notes corroborated) and that I generally drive responsibly and am not a habitual speeder, so the prosecution agreed to seek the minimum - but for 85 over they will likely seek a high fine and a long suspension if not jail.
They will NOT plead this down to a lesser offence. Unless there are fatal errors on the ticket and/or glaring holes in the officer's notes (which is VERY unlikely for a stunt driving charge), IMHO your best bet is to hire a good and EXPERIENCED lawyer who can hopefully convince the Crown that you're generally responsible and that you've learned a lesson and arrive at a sentencing recommendation BEFOREHAND, which they will then ask for (and likely get) from the Judge.
If your arraignment is at the court in Newmarket at 465 Davis Drive, there is someone I can recommend to represent you if you're still looking.