850Turbo,
You can put any grade of gas in the car as the computer will adapt and make changes (knock sensor,oxygen sensor etc) for best efficiency. The average driver won't even notice a difference. But hey, we're not average drivers!. If you can afford to put in higher octane, then for sure it is "better" for the car. The performance will be marginally improved, but more importantly, the engine will be burning a cleaner, leaner fuel that your injectors, plugs and engine will benefit from. As far as plugs go....850s (especially Turbos) are fussy with plugs and demand plugs in new or like new condition. Having said that, I put in the cheapest gas I can find in my 850 and have done so since purchasing it five years ago....no problem.