The er4p is the same earphone with lower impedance. This makes them easier to power without an amp, but also attenuates the treble. So they sound more muffled compared the the 4s, however this makes them a little bassier as well. But even the 4p is better than most iem in the treble.
I don't know if they're a league above the ba200. Especially value-wise, but I do think they're the best I've ever heard. I think earphones only have small differences in quality as they get more expensive past
$100. I find its more about frequency response, which I would argue is quality, but some would argue is preference. If you don't want neutral I don't think you'll find the other traits differer greatly. Most things like soundstage and depth may differ but are greatly affected by frequency response. So while there are differences the biggest change will be the sound signature of the frequencies.
Hmm ok good to know :)
I don't really agree on the 100$ price point, at least not for full sized headphones. I think realistically it's closer to 400$. That's the price of the HE-400 and technically it is clearly a couple of leagues above all other headphones I own/owned.
You said you wanted to make custom sleeves? It's pretty pricey right?