I got my plugs from
https://burgertuning.com/ They sell a kit with a thin walled socket that you'll wish you had when you get things apart and figure out that your sockets don't fit too well in there. They also have a nice gapping tool if you plan on running tunes you will likely need to re-gap the plugs. It very well could be that the prev owner had tuned the car and gapped the plugs for the increased boost.
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2015 F06 M6 Silverstone Metallic II, Individual Full Leather, B&O, Night Vis, CCB, RKAutowerks (CP Rods/Pistons, DI Cams, CF Intakes, Coated DP's & Billet Diff Brace), InfinityAuto Turbo Inlets, Mishi Oil Cooler, JB4, XHP S3, BM S3