Tune will be best value for the money on N51, since it unleash the hidden power of 3 stage intake manifold. your XI definitely need that extra 20ft/lb torque at 2500-3000rpm for daily driving.
I would probably get Tune first then try find used PE-PI combo alone the way, the tune will adopt to any mod you add on afterwards.
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N51- PI - PE - ESS Tuned - SSK - CDV Delete - AFE Header - B12 - Mfactory LSD
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