Let me see if I can help:
- Customs sound much better than any universal I have owned, and I have owned several top tier universals
- Supposedly more drivers can make a custom sound more effortless, or produce more tactile bass, but no one that I know has taken a JH13, for example, and disconnected half of the drivers and then compared with a fully functioning JH13. The JH16 uses additional bass drivers, but it is also tuned differently.
- Hard to compare apples to apples here. Maybe a UM reshell where an additional driver has been added, but then again there is no A/B testing and it is really difficult to tell without doing that IMO.
- Making customs takes several steps, check out this article.
There is a law of diminishing returns, so spending double the cost of the SE535 isn't going to get you double the performance, which is usually true with just about everything. The base TV vs. the top of the line isn't all that much better in picture quality, but it is better! The most important thing with customs IMO is to get a sound signature that you will like as there is no sense in getting something that may be technically superior but you don't really care for the sound.