I wrestled with this too. I have soffit vents and gable vents working with an exhaust fan. here's my conclusion: One often-overlooked reason for a vent system is to keep the roof temp down to be shingle friendly. More heat "boils off" more beads and helps kill the asphalt base. In 21 years my roof is aging *very* uniformly and has lasted way longer than most neighbors. I take this as evidence of equal cooling.
The German in me would have me take balometer readings (HVAC flow meter) at every vent to be "perfect"-- but I'll live with the real-world results.