I think that the thing so many of us forget is just how difficult it is to get acceptable translations between even bona fide XG and GS soundsets from actual Yamaha and Roland gear. As Rikki points out, there's a LOT more to it than simply getting the right drum sound on the right note number.
Even with the simplest non-velocity switched drum sounds, there is the issue of velocity CURVE, which is only further complicated by newer drum sounds, with multiple samples per note. What is the crossover point of these samples? There's no hard and fast standard, even the Big 3 themselves have no consistent crossover points, nor velocity response curve.
Next is drum EQ... No consistency here, and so on, and so forth. Let's put it this way. It is notoriously difficult to get translated styles to sound as good as the original, and that is with the benefit of at least a fully balanced sound set like you get in a Big 3 product... Now you want to do it on GIGA sets that, IMO are anything BUT consistent and balanced.
It's an uphill battle, that eludes the best of us... Even Roland, to be honest, have a far less balanced soundset since they abandoned GS Sound Canvas based products. Sure, some of the newer sounds are formidable, but the overall consistency of sounds has suffered. And if these guys are having problems, what is the chance mere mortals like us can succeed..?
Just designing ONE sound that impresses is tough enough. Now do that for at LEAST a couple of hundred (what you need for full-ish GS or XG compatability), and THEN make sure that they are all interchangeable (change a bari sax for a tenor one, and have NO volume, response curve or EQ issues - change one drum kit for another, and have NO issues with drum sounds louder or quieter, consistent velocity cross-switch points, or EQ issues), and you start to realize the gargantuan task that faces you...
Now add in Mega voices (after all, what's the POINT of playing Yamaha styles if you don't have the one thing that makes them so good?) and the weird things they need the OS to do (like only transposing within a certain note range and leaving others untouched, all on one part) and the deal gets even MORE complicated.
I hate to say this, but soundset design is a task for professionals, and even THEY have problems...
And all this, just to be able to play PSR styles when a cheap PSR (comparatively) plays them perfectly already. Seems a case of diminishing returns, IMO