Iirc the reason browsers don't have knuth-plass justification is because it runs in O(n^2), but even so my research tells me justification reduces retention because your mind retains the "shape" of ragged text. I don't have the article on hand but if I find it I'll add it later.
A bit late but [1] shows equal comprehension between justified and ragged-right but much faster reading speeds with ragged right (post-stroke patients in Russian). [2] shows slightly faster reading speed with justified text but less comprehension on a delayed test (also in Russian). [3] has no empirical data but cites articles and claims comprehension is equal between the two formats but ". Poor readers, however, read unjustified text more quickly than justified text."
I would say it's not overwhelming, but I'd go with ragged-right to be safe.
