i think the more important issue here isnt the transmuting effect but with backstab itself. you have correctly determined that assuming 100% SA, the dps is greater. like what you have described, 100% SA is impossible to attain and gave several reasons. but there is 1 more reason, which is given the dps nature of the monster, it is almost impossible to lose agro (unless diplo, someone intim or radiance)
also another factor is khopesh is not a good weapon to have radiance on since it crits less than a rapier. in my experience, i do not think i ever saw a radiance khopesh being crafted. strength base khopesh weilding rogues are more likely to have mineral 2 instead or even lightning strike
ps i dun think raid bosses can be blinded but i could be wrong here
I have seen several radiance khopeshs crafted, I belive Katsumi or Serpentwave was the first to craft a radiance wepon.
You are right, raid bosses are immune ti the blind effect but can still be hit be the light dmg and sneak attack dmg.