Phase plug materials for B&W 803D/diamond

I was reading that people have purchased uncoated aluminum phase plugs for the FST midrange drivers on various B&W 800 series speakers. Some have touted these as "upgrades". I have a pair of 803Ds (first D version), which comes stock with "black" phase plugs. At first I thought these were plastic, but after removing them, I found they were actually powder coated aluminum. B&W also sells uncoated aluminum phase plugs for various 803 d versions. Some claim that using the uncoated Al phase plugs alters the inductance on the midrange drivers and "extends the midrange". Supposed to have something to do with the metal/metal contact between the pole piece and the phase plug. The stock powder coated phase plug is isolated by the powder coat (except for the metal mounting stud) which is non-conductive, whereas the uncoated Al phase plugs make metal on metal contact over the entire rear surface. Because the voice coil overlaps a portion of the phase plug, this metal on metal contact (or lack thereof) impact the inductance of the driver.

I tried simulating the uncoated Al on mine by machining off the powder coat on the rear, and the results were far from pleasing. The speaker was noticeably brighter and almost unlistenable on some program material. I ended up putting an insulating washer back between the phase plug and the pole piece which restored the natural balance. A am also wondering about the impact of the mounting stud, which on the stock plug is magnetic carbon steel. I wonder if using a non-magnetic material like 316L stainless or Al or Titanium may also have an effect?

I am curious as to whether anyone else has experimented with different phase plug materials and their results.