I use a 10 gallon kettle, my boil volume is usually around 7-7.5 gallons. I try to end up with around 5.5-6 in my primary, which then allows for fermentation loss, trub loss, transfer loss, and keeps me at or above 5 gallon bottling volume.
IME, a very high gravity beer that starts out with a pre-boil volume around 7 gallons or 7.5 will sometimes result in some boilover in my 10 gallon kettle, little heat monitoring and trial and error to reduce these, but I don't think you would have a problem with a 9 gallon. I just say all this to say If buying a new kettle, If I were doing it again, I would go with a 15, which would allow me to not worry about boilover at all, virtually, and then I would have it should I decide to do a 10 gal batch.
Edit: I missed the part where you had said you had bought the kettle, dumb me. Have fun!