I thought a stellium was any amount over 2. Wow.. that is something. I don't know that it would have -more- of an effect than the sun sign, but they will definitely have a large effect, I think. Does this person act a lot like a Capricorn to you? Do you think the person has more Capricorn-ishness to them than Aquarius-ishness?
In my opinion, we need to look at the entire chart before we understand how the entire chart works. A six planet stellium in Capricorn (duty) is clearly important in the chart. It would be come even more important if there were a focus on Saturn or 10th house (both have to do with duty).