Sunny 16 applies to films of any speed.
It is not unusual for a meter whether incident or reflective to disagree with it. Like all other rules in photography, rules are meant to be broken. Clouds, atmospheric haze, smog and other abnormalities in the air will alter the amount of light falling on the subject. If your negatives please you, don't worry about the meter. If the readings are close to the Sunny 16 rule you are OK. If it begins to vary a lot then you need to worry about it.