Is there a limit to how much MPC can be added to Greek-style yogurt?

Yes. Some practical yogurt industry work has shown that, in combination with proper stabilizers, MPC can be used to fortify 3.2% protein milk up to about 6.4% protein yogurt milk without a corresponding loss of gel smoothness. At fortification levels above 6.4%, resultant yogurt gels start to become grainy in texture, losing the smooth mouthfeel that is desired by consumers.