Yamaha, Roland, and all the rest of the big players have had decades to develop sounds for their keyboards. One of the advantages of this (particularly as they started sound development WAY before RAM was plentiful) is a huge library of well developed sounds that don't take up huge amounts of memory. 4GB of sounds sounds like a hell of a lot, until you realize that maybe a GB of it is just for a piano sound, and so on....

The amount of RAM a sound takes up will help a sound get more realistic, no doubt about that, but especially for live use (that's what an arranger is all about, after all) it's all about the QUANTITY of good enough sounds, not how stellar a few can be.

Yamaha and Roland probably put more money into just sound development than Lionstracs or Wersi's entire R&D budget. The results are instruments, particularly at the top end of the line, that for live use are close to perfect, deep, rich pianos, expressive saxes and drumkits that POP!

Could they be a bit better....? Of course.
Is the difference worth the hassle...? If you like tweaking (not a lot of tweak-heads playing arrangers).
Is the difference worth the extra money they cost....? That's what the market is deciding.

Once again, don't get me wrong.... I think these things are the future. I am just not brave or rich enough to make them my present (Xmas or otherwise!).

An arranger lives and dies by it's styles. The sounds are just lagniappe. Witness how many bemoan the Technics line, who's sounds aren't up to contemporary comparison (mostly!) but are missed by all who used them for the styles, mostly...

Roland's sounds, in many categories, are not as good as Yamaha's. But the styles more than make up for it (if you like the Roland 'approach'). The T2 would sell much poorer if the styles weren't as good, despite how good the sounds are....

Anyone building an arranger, rather than a workstation, had better put kick-a$$ styles as priority number one..... No amount of Giga-quality sounds will make it sell if the styles don't inspire, and in large numbers....

Imagine how poorly the T2 would sell if it came with few good styles, and you were expected to develop or translate them yourself. At that point, you probably wouldn't care how good the sax sound was!