You can end up chasing issues like this for quite a while.
It looks like the surface is overheating a bit and the rubber is starting to superheat. Naturally Pirelli's have a tendency to have this behaviour moreso than other brands as temperature rises, but it looks more prevalent than usual. It does look consistent across the arch however and we don't see any obvious tearing. Usually if you are overheating due to bottoming or incorrect spring rate, you see it on the edge where maximum load occurs, but both the center, driveline and edges are relatively consistent. I'd be curious if they feel greasy, in particular when you get on the throttle driving out of corners? Do they feel greasier as the session goes on? As suggested a bit more pressure would be where I'd start.
No offence to TAD's post, but that tire looks odd, as though its somewhat underdampened and is pogo-ing in the corners given the inconsistency in the wear causing some tearing.
Of course your suspension is going to play a big hand in this. I can't quite tell from the picture what it is, is it stock?
If that's the case, its possible the shock is starting to fade as it heats up.
If you are looking for data, here are a few pictures for comparison using a K-tech 35DDS, on a rough track with an SC2 that I would argue is pretty close to ideal. Usually there is a bit more smear on the tires, I think it was a cooler day. I'll do 2-2 1/4 days on a rear, which is consistent with other brands, mid pro-level pace. Given the price of tires, strongly consider having that shock looked at or rebuilt. It will cost you a bit for a proper shock/service, but eating a tire a day gets spendy. Start from a known quantity and adjust from there. Also look at your spring rate/weight.
We've all chased setup, its great when you get it, but frustrating when you can't