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I would look at pulley sizes on the kits. IIRC they both use Vortech V3 Si units, so the same pulley size should achieve similar results.
The VF bracket is superior. I know of several ESS owners that have had oil filter housings crack, including the previous owner of my car. The VF kit has more belt wrap for less slip. I believe ESS redesigned their bracket a couple years ago, but I haven't really followed.
I haven't heard of either VF or ESS offering alternative software for the Z4M... only the E46 M3.
554 whp sounds like a lot of dyno correction factor or a very generous dyno, since it would mean the engine is operating at close to 200% VE (almost 15 psi of boost with zero supercharger drive losses or ignition retard due to heat). I wouldn't trust a canned tune at those boost levels on an S54, no matter who it's from.
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