Diacetyl isn't reduced as a gas, so pressure fermentation doesn't have much of an effect. What does matter is whether your yeast can continue to work efficiently under pressure. With saturated CO2, yeast has a harder time getting things through its cell walls. Certain strains do better in pressure fermentation than others.
You can take a sample of your fermenting wort and see if it finishes with a different level than what's under pressure.