The lower power models can surely be expected to provide the same acoustic and electrical performance at <300W, which is the main power range of concern to the vast majority of DIY PC builders.
I wonder where you get this from? As we've seen time and time again, fan ramp ups are not the same for PSUs of the same series with different power ratings. Ones rated for lower wattage tend to ramp up faster under load, since they have less headroom. It's only overengineered PSUs which are essentially high power ones that have been marked down that don't tend to do that.
The sweet spot for silencing for the TP-750 is 300W load; if you can achieve the same results using lower rated models, they would undoubtedly be a better buy since they'd be cheaper and you wouldn't want to buy the TP-750 for silencing if you're going over that anyway. A test to confirm that would be very nice, rather than just inferring that they'd perform similarly based on a PSU that's more powerful than most single-gpu users will ever need.
It's often the case that in a line with say 5 to 7 models they will do this:
small heatsink lower rated caps
big heatsink lower rated caps
big heatsink higher rated caps
each strategy might be marked as 1 to 3 output numbers with the same or more cables/connectors.
In other lines they will avoid the heatsink change and just change the caps.
In cases where the fan and heatsink are the same noise isn't affected by a change in caps. So you can easily say in those cases that a 550 and 650 would sound similar (manufacturing variance is always an issue that keeps you from saying they sound exactly the same).
The only major exception to this rule of thumb worse than minor heatsink changes is a line like Corsair has where they have completely different internals from multiple companies under the same brand name (Seasonic and ChannelWell Technologies aka CWT). In those cases a 550 and 650 might sound totally different because they are wholly different designs with nothing common between them (diff PCB, fan, caps, heatsinks, etc)