Who uses a streamer as a NAS? For those users, gigabit may actually be a benefit indeed, that's if the 8646 actually offers decent speeds (-> gigabit is more or less a gimmick as far as the WD Live Hub is concerned, for example, since the (albeit older and weaker) Sigma SoC is nowhere near powerful enough to make use of it). But as far as the streaming itself is concerned, as explained earlier, gigabit offers no advantage even with today's highest bitrate content. It may have been different if the 8646 offered full 1080p 3D support, which it doesn't.
The GUI of my 8642 based Neo TV550 is anything but slow. The Dunes aren't slow either, even with the pretty extensive external media management solutions like Zappiti. Very few people use the new Popcornhour NMJ interface, which is slow indeed, since there are good alternatives like YAMJ.
The Popcornhours aren't the only 8642 based devices. If Syabas aren't able to implement a proper power off function, which other companies have, they should be blamed instead. I actually agreed earlier that standby needs to be improved, but again, this is hardly a feature that makes the 8646 worth waiting for, especially if you already own a device that can be properly shut down.
Die shrinks cost money. The 8646 has a higher clock speed (800Mhz vs. 667Mhz of the 8642) and 256KB L2 cache. For that, you don't necessarily need to shrink the die. The 8646 is basically fully optimised version of the current architecture.
The 8910 will be based on a completely new architecture, offer 3D Bluray support and considerably higher performance, especially as far as the 3D graphics acceleration is concerned (making a port of XBMC feasible, for example). I don't know if there will be a die shrink since I can't look in the future, but it seems likely.
However, one area of concern as far as the new Sigma SoC generation is concerned is the more extensive support of DRM, Cinavia in particular, which is likely. While this is in stark contrast to an open platform like XBMC, Sigma could be forced to implement it by the time the devices hit the market. But that's all speculation for now, time will tell.