I can't speak on textbooks for the Kindle from the publishers, but I can say that the DXG has been wonderful for me as a visually impaired person. From left to right, there are eight font sizes, with the font on the left being the smallest and the one on the right being the largest. I believe before the 2.5 update, there were only six fonts and Amazon added two much larger ones. When I had the Kindle 2 before the 2.5 update, I was reading at the largest font size, now with the two additional larger fonts, I have moved up one and it is much more comfortable. I'd like to see Amazon add two even larger fonts for people with extreme low vision and bring the variable font sizes to the home screen, menus, dialog boxes, and Kindle store. I would also like to see an inverted display mode where the text is white and the background is black. Of course, this can all be done through firmware. Hopefully they'll get around to it some day.
However, I would keep an eye on Pixel Qi and some of the other display companies who are working reflective display technology. Because once these new display have been perfected and proven, then they will show up in just able everything including netbooks, tablets, phones, and maybe even your electric toothbrush and toaster. Personally, after seeing what a couple totally blind friends of mine have done with their netbooks and the accessible Kindle for PC app, I think it makes for a better option in many ways. Especially when you are looking at students. The netbooks have a touch typable keyboard for note taking whereas the hardware Kindle ereaders do not. In a few years, we should be seeing very good battery life on netbooks and tables with the reflective display technologies which will become standard in them. The battery life may not completely rival the Kindle, but it will get an owner through several days if not a week with the backlight tuned off and the hard drives being replaced by flash memory or solid state drives. Keep in mind, that when you are considering the Kindle DXG, a netbook isn't really any different in size one you add the cover to the DXG sized ereader, so why not go for the touch typable keyboard and also have a device which can make other aspects of a low vision students world more accessible? Anyway, let us know what you learn over time.
BTW, when was the last time, the DX and DXG received an update? It seems to me, it has been quite a while since we have seen one.