The WDTV HD Gen1 uses the Sigma 8635 processor @ 300MHz.
The WDTV HD Gen2 uses the Sigma 8655 processor @ 500MHz.
The WDTV Live uses the Sigma 8655 processor @ 500MHz.
The WDTV Live Plus uses the Sigma 8654 processor @ 500MHz.
The WDTV Live Hub uses the Sigma 8654 processor @ 500MHz.
The only difference between an 8654 and an 8655 is that the 8654 is the DRM version. That’s why NetFlix could not just be added to the WDTV Live, and WD had to build the Live Plus with the version of the chip that will play the protected streams.
So, except for the original WDTV Gen1, all this family has the same processing capability – the Hub doesn’t have a faster chip.
The local storage hard drive is pretty much needed for the added capabilities of the Live Hub… there really isn’t any “something” between the Live Plus and the Live Hub. It either has a place to store the info it needs, or it doesn’t.
And I’m fairly sure you’re going to end up with most people saying they’d prefer WD to work on existing bugs in the Live/Live Plus/Live Hub, rather than release yet another player in the same family.
As for FF/REW, I believe the core playback code is essentially the same for all devices, so if your encodes aren’t working properly, they’re going to behave the same on the whole WDTV line. Most people don’t have any FF/REV issues with their files, though.