I've got mixed feelings on this subject. 1) part of me is like hell yeah MORE POWER!!! 2) but the other part of me is like wait wha!? See these Primaris are kinda like a cross between Custodes and Astartes to me. In the Heresy novels they do a pretty good job of stating that the Custodes are better then any single Marine, but the Astartes strength lay in group and team work, built into their DNA. Custodes tend to be WARRIORS, Astartes are SOLDIERS. Anyone who has every been a soldier/police officer knows the difference. So yeah, now you are basically taking the strength of the Custodes and the strength of the Astartes and combining them with no future limits. Why would you want regular Marines?