IME when splitting the relative output drop is about the same regardless of how hot the humbucker originally was.
Also IME, as far as sound, both a Custom (any flavor, Custom, Custom 5, Custom Custom, 59/Custom) and a Black Winter when split and sounded very Fendery. The Custom family almost quacks on it's own when split to the slug coil with normal orientation along the string (think Dire Straits Sultans of Swing or Clapton Lay Down Sally). It sounds right and mixes well with singles and stacked singles. I have a StagMag also and I wouldn't say it was closer to Strat than a Custom, just a different flavor of Fendery sound, little more rounder but snappy kind of sound (or in other words every so slightly darker/softer on top but somehow a bit twangier than the Custom).