A couple of things that stuck out in analysis right off the bat:
1. I wasted 150 points in my Army List. I paid 250 for Mephiston to stand in one place and cast Prescience each turn. I could have instead paid 100 points for a stock Librarian to stand in one place and cast Prescience each turn.
2. The Plasma Cannon Devastators felt very expensive to me. It was painful for me to field them in the beginning. Add to this the fact that they failed morale and ran off the table after doing nothing, and I have a real bad taste in my mouth.
I don't like to stand in one place and shoot at the enemy. I far prefer closing with them into melee. However, shooting does seem to be king in 6th edition. I do not think that it is to the exclusion of melee. Nevertheless, each army needs to find the proper balance between the two, specific to race and ability, in order to be strong.
I need to conduct more research!! More games of 40k are in order!!
I would like to conclude by saying that the most important part is that fun was had by all involved. It is always a pleasure to play a game against Dark Saint.