I've tried all of the current 8" tablets for extended periods of time for reading. I own (or owned) the mini, the mini retina, the LG G Pad, the Samsung Tab 3 8", the Samsung Note 8", thus Asus Memo HD 8" and currently have the Tab Pro 8.4 on order.
I also had other slightly smaller tablets like the 7.7" Toshiba AT270 and the Samsung 7.7"
They all work fine for reading. Weight with a thin 8" tablet is negligble. In fact if you compare any of these to the top end reader available, the Kobo Aura HD, they only weigh on average about 100 grams more. That is not a big weight delta.
The Samsung tablets (the tab 3 8") and the new Tab Pro all have a reading mode. I used the Tab 3 8" for about 6 months and left it on by default. Wasn't really sure how it helped (or if it did at all).
Two things are important for reading under various conditions, max brightness/dimness and DPI. Max brightness is important if you are reading outside (one of the main reasons I got rid of the LG G Pad), max dimness is handy if you are reading in bed in the dark and don't want to wake up the wife with an 8" flashlight...
DPI, the higher the better as it increases the sharpness of text. As a reference, the Kobo Aura HD is 1440x1080 (6.8") with a DPI of 259. The Tab Pro has a 359 DPI....