Last time I changed my plugs I went with 7 & 9 because the car was stock. I'm getting ready to do a midpipe/ecu/boost controller and change the plugs so should I go with all 9's or maybe 8 & 9's.

My city driving is about equal to my highway driving if that matters.

 

I'd go with the 9's all the way around, they are colder than the 7's and will help prevent detonation a little better. They will foul easier, but just check them often, and don't clean them with a wire brush if you buy the platinum plugs, this ruins the platinum coating. Just wipe them off with some carb cleaner and blow them dry with compressed air. Just don't go crazy and buy the race plugs for daily use, they foul REALLY easily and they're not cheap. Maybe get a set for the track but that's it. Hope that answers your query.
Art

 

Yea, all 9s. I would say maybe 7s or 8s if you lived someplace cooler but Texas is pretty warm right I have more info on my site regarding different plugs.

 

@ what point in upgrading can you go with all 9's?

 

That's hard to answer because it also depends on other factors like wear you live (hot cold) and how you drive. I'd say if in doubt go with all 9s and if you notice fouling too soon go with a warmer leading plug.
cvs

 

BUR9EQP

http://www.mazdatrix.com/ign-2.htm

 

You can go with all 9's even if your car is stock. It really doesn't make that big of a difference. It may slightly rough up your idle, and you will lose a couple horsepower from running colder plugs, but that's all pretty negligible.