Will there be a performance benefit in the upgrade for iPod touch 5 to iPod touch 6 for the KNFB reader?
There is no iPod Touch 6. In fact, the iPod Touch hasn't been updated in 2 years and even at launch did not have as good a camera as the the iPhone 5 it was introduced with (the iPod Touch camera is coverable to the iPhone 4S or maybe even the iPhone 4?). I can't speak about the KNFB reader specifically, but the newest iPhone (6 and 6 plus) are much faster than the iPod Touch 5 from 2012, so no, the older hardware in the iPod Touch 5 cannot provide the same level of functionality.

Whether the functionality in the ipHone 6 is enough of an improvement to make the cost worthwhile is a question only the user can answer.