The Flat4-SUI is one of my absolute favorite IEMs as well. TBH, you're not going to be seeing drastic improvements even though you'll be spending twice as much. Diminishing returns certainly apply. Anyway, here's my thoughts on some of the ones you've listed which I own:
Final Audio FI-BA-SS: Very refined, but has a certain ruthless or raw quality to its sound in that it'll reveal everything in a given track, flaws and all. Despite this I wouldn't characterize it as harsh. Treble is energetic but well controlled like the Flat4. Bass is less emphasized, but extremely well textured and punchy. Detail retrieval about on par, however presentation is more externalized and less "in your head" compared to the Flat4. Analytic but natural sounding. Comfortable for me to wear and beautiful to observe, you'll want to be careful when handling these as they can scratch easily. Mirror finish is also annoying to keep polished (you'll probably want to, at any rate).
Final Audio Piano Forte IX: Extremely mid-centric sound, with rolled off bass and treble. Midrange is warm, liquid, and the overall sound is very "vintage." The VIII, IX, and X versions sound more alike than not, however the X is the most extreme in how it's voiced. The IX is a bit more balanced, though that's in a relative sense. These are very forgiving but sound more involving on some tracks than others. Presentation is extremely expansive and hall-like, but the sound remains very thick and blanketed. Ergonomics are very hit or miss: there are no tips aside from the built-in metal flange that is one-size-fits-most. Very shallow insertion, so not suitable for strenuous activity.
FitEar TO GO! 334: Very balanced sound, though there's a slight bass emphasis. Sound is refined and polite, with an emphasis on "getting out of the way" and allowing music to come through as it was recorded. More of a monitor-type earphone than a strict audiophile device, so it may lack that immediate wow-factor for some people; however it gets very little wrong in my opinion, and due to its transparency it's something of a chameleon in that it changes from track to track. Some have issues with fit, but I find they fit extremely well, even better than my full-on CIEMs. Cable is extremely microphonic. Build quality however is top notch, perhaps the most impressive of any in-ear monitor I've encountered thus far.
Ocharaku Flat4-KAEDE: The most impressive presentation and packaging of any in-ear monitor out there in my opinion. Basically a special edition of the Flat4-SUI. Sonically it is more similar than dissimilar, with perhaps a very slightly less bright and more bass-emphasized sound signature relative to its predecessor. If you already own the Flat4-SUI, I'd consider the others on the list first.
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I'd also add the following to your list of considerations...
AKG K3003: A very airy and open presentation, though it can sound a bit unfocused and incoherent at times on certain tracks. In terms of tonal balance, I find the standard reference filter to be quite satisfying overall. Highs have a sparkly quality and sense of air that make this sound very "hi-fi." The midrange gives vocals a breathy and ethereal feel. Very comfortable for me to wear and strongly built, though I'd advise against the version with an inline remote as some have reported failures with that version (the "K3003i" version).
Sennheiser IE800: My new favorite universal. This thing has some of the nicest mids of any IEM I've ever heard, universal or otherwise. Biggest issue is the bass which is too emphasized on certain tracks for my liking, though it's a clean bass and doesn't muddy-up the midrange thankfully. Highs are refined and well controlled, sounding relatively linear. Overall presentation isn't as airy as the K3003, but it has more focused imaging to my ears. Cable is too short, but the overall build quality seems robust.
