But think about it logically. Why would it be the case that the Primacy offers better grip than the Pilot Sport?
The Primacy is a mid range Michelin tyre designed for regular cars, not performance cars. The Pilot Sport 4 range is designed for performance cars - it literally prioritises things like grip and feel over things that are perhaps more important on the mid range tyre like fuel efficiency and life span. Both are made and designed by the same manufacturer to fit into different parts of the market.
Lets look at what Michelin say about each tyre:
I mean come on - one talks about fuel economy and tyre life, the other is clearly designed for ultimate grip at the expense of things like tyre life.
I guess only you know what you've experienced but you have to admit it's a strange finding, right? Why would the tyre they designed to for going shopping in a Ford Focus offer better grip than the tyre they designed for fitting to a performance car?
I have absolutely no bias here - I have both of these tyres on my cars - the Primacy and the Pilot Sport... it would be great of the Primacy really was better than the Pilot Sport as its cheaper
I have to wonder whether what you are really experiencing is the difference between a worn tyre and a new tyre. Otherwise why would you have been in a position to be able to directly compare them with the same sort of wear levels etc? How did you even get to the point where you've bought a set of Primacy for a 128Ti anyway? Why did you pick them over Pilot Sport 4 (Or a similar tyre from someone else)?
There is of course nothing wrong with a Primacy. It's an excellent tyre and you can probably make a case for fitting them to a 128Ti. But that case wouldn't and shouldn't include 'because it offers superior grip to the Pilot Sport 4 range'.