It's all still very inexact and subjective. Even staying within one manufacturer, one could say the W4 and UM3X are equally good as far as a certain level up the chain but they sound very different from one another.
If somebody is into music and into good audio I can't imagine somebody buying one or two IEM's and saying "this sounds great, I am done." It just doesn't work that way with any hobby. I don't know any fisherman with one pole, or photography buffs with one camera or hunters with one gun, etc.
Improvements are relative to one's budget. Let's say you are a really big fan of SE535. Of course the bass enhancement of SE846 is NOT worth more than double of what the SE535 costs but if you have the money to spend on an upgrade and you feel that a much improved bass was all you needed to reach your audio nirvana, then by all means it is worth it.