As a PSR-E433 owner, I can assure you that, even though it is a newer model than your 550E, at a street price of $280 USD, it is still a step or 2 down from your 550, even at its age. Most of the E-433's technological advances were in the addition of the DJ rhythm features and some updated styles (The reasons I bought it). Some of the tones seem to be improved, but that is subjective. Even though it recognizes most valid three and four finger chords, it will not handle the more esoteric chords you are thinking of. You will probably need to go to a higher priced unit, but even at that, you have to be very careful. Equipment manuals are not much help when it comes to this topic, as they prominently list the common chord types in the main text, but the more complicated chords are always covered in an exhausting list of barely understandable footnotes. It has also been my experience that, for features like this, a limitation in the BOTL model carries all the way through the model line, all the way to the top and seems to carry over from one model year to the next - kind of a "one size fits all - forever" engineering in this area. I guess the assumption is that jazz fans don't play arranger keyboards. So just buying a higher priced unit is no guarantee of better performance in this area. As an example of what you can run into: I took a piano chord dictionary and started running them through the arranger section of my Casio WK-7500, and really got some surprises. The first was that a simple flatted 5th produces no sound at all from the arranger section. Neither does an Add-5 or an Add-11. It will play an Add-13, but it plays it as an Add-6. It will play an Add-6 or an Add-9, or even an Add-6,9 just fine, but it plays a 7th-Add-6 as a common dominant 7th - and these are all relatively basic compared to what you are thinking of.
My point is, you need to do exactly what you have done here, ask for advice based on hands on experience, but you need to ask in other places as well. For Yamaha gear, I would recommend the PSR Tutorial Forum - particularly the PSR-S750 and S950 Forums. Check the Korg forums for Korg gear. Roland has not been a serious contender in the full-blown arranger arena for some time, so I doubt that you would find much help there. Perhaps the best would be to post your question on the synthzone.com General Arranger Forum. Stay out of the specific model forums, as they see very little traffic. Everyone there hangs out in the General Arranger Forum and most of these folks are long time (life time ?) pro giggers, so if anyone can steer you straight, they should be able to.
Good luck !
