The number one enemy of a shingled roof is poor ventilation, esp on vaulted roofs. Vaulted roofs should have insullation trays installed prior to the actual insulation. Soffitts should be properly vented, gable vents are a big plus and an attic ventilator is an even bigger plus.
It gets so hot and humid here that 'continous soffitt' venting is common. Some even put ridge vents on along with gable vents. I am not a fan of ridge vents, unsightly and an avenue for water penetration. I do have continous venting in the flats of my soffitt, gable vents in every gable, one ventilator and I plan to install another next spring, R-39 insullation, and "reflecta shied" roof decking.
I have GP 30 year Onyx shingles, 5 years now, still look like new. I had the same shingle on my last house but it was build with no gable vents, not enought soffitt vents, and a ridge cap. The shingles were faded after 5 years and the new owners recently replaced the shingles, less than 15 years on a 30 year shingle. Fungus was a horrible problem on that house as well.
I just don't believe that you can too much attic ventilation, within reason.