I have questions. What is your climate? Are you in a climate given to cold winters? If you are, are you experiencing problems, such as snow melting on the roof & creating ice buid-up?
The reason I ask is that many people look at ventilation as the "Holy Grail" to solve such issues. It's not! In fact, adding more ventilation, especially power venting, could make matters worse (Power vents, this includes turbines, should only be used in hot climates where the objective is to get a lot of heat out of your attic. Using Power vents on cold climates creates a negative pressure in your attic, exaggerating any air leakage from your living space into your attic, carrying more heat into your attic).
If your experiencing problems with the amount of venting you have, something else is wrong. Solve the problem, not the symptomn.