Haven't played or heard such a piano live, but I can make some “educated guesses”.
The most clear-cut difference is that longer strings have less inharmonicity. Thus they will sound clearer / cleaner, and reduce the need for stretch in the or even make it unnecessary entirely. The only flip side of this is that it will sound in a way “too perfect”, perhaps sterile.
All other effects will come not from the string length alone, but from a combination of it with other factors. Indeed that's also true for electric bass – the article claiming that short-scale basses sound inherently fatter is frankly a bit rubbish. Both short- and longscale basses have fundamentals plus harmonics in similar proportion. The differences are
- Short strings have a quicker decay, in particular quicker decay of the higher harmonics.
- Shortscale basses will be operated with thicker strings and/or less tension, and often higher string action. All of this facilitates that the player can more easily put a lot of energy into the strings. On the flip side, you can't put in as much energy before the strings start rattling against each other and/or the fretboard. Thus, a shortscale bass has in a sense some dynamic compression built-in, which is well known to help with “fatness”.
- Longscale basses have more stable and clear-sounding harmonics, which encourages bringing those out in the EQ. That however may end up overpowering other instruments in the mix, and if the result is that the bass volume will be turned down then you also lose “fatness”.
- The inharmonicity aspect can also be perceive as some “fatness”. It's kind of a bit of a chorus. In particular when playing over a saturating amp, you'll get both the detuned original harmonics and overdrive harmonics which are exact integer multiples. I.e., a shortscale bass has in practice not only slightly different harmonics but actually more of them. Of course, exactly that also contributes to them often coming out muddier.
Unlike with electric bass, on piano you don't get to apply EQ to the signal as you like. However, the instrument itself acts as an EQ too, and specifically the large size does make it easier for the body to bring out the low fundamentals. Also, more so than with bass, a piano can be designed to put much more energy in such long strings. So I'd expect the 450i to sound significantly more powerful in the bass register than a normal grand, certainly not “thin”. It may also have such an open overtone spectrum that it becomes overpowering – however unlike with turning down the bass in the mix, simply playing the low notes at a quieter dynamic level will also make them sound rounder, fuller, so that should never result in a “thin mix” either.
And because a piano isn't played through a nonlinear amp, the “multiple extra harmonics” aspect doesn't apply at all.
In Pianoteq, you can freely vary the lenght of the simulated strings. At least there, it affects pretty much only the inharmonicity amount. That may not be quite realistic, but you can give it a try and decide whether you feel that eliminating inharmonicity does in any way take away from the “piano quality”.