You'll get many different responses on this one but I hear a lot of people telling me to just stick with the OEM Bosch copper core plugs on 850 Turbos. As for the 850 non-Turbo, I'm not too sure but I heard that platinum plugs work very well in that model. It's highly unlikely that you'll get any appreciable gain in terms of performance just by switching to a different brand/type of sparkplug. Also, in case you're wondering, sparkplug replacement schedules on 850s are already considered by many to be too long already--I change the plugs on my 850 Turbo every 25,000 to 30,000 kms.