I had a problem with the CA flush, but that was because my FPR went bad in the middle of the flush, and I could not get it all out and it sat in there two days and rotted out the freeze plugs.
To the greater question of overheating: My car was overheating this summer and the block had just recently been tanked, and was free of any corrosion inside of it. I then replaced the Tstat with the 70C one you are referring to. That did not help at all. I then went and ordered a new radiator AND a new condenser which sits right in front of the radiator, and since then my car has not overheated once, even with the AC blowing at full clip.
I live in one of the hottest cities in the US, Lancaster CA, where the summertime temps average over 110F from July through September. Not once after then did the temps go over 100C, an acceptable summer operating temp.
When it did get up to 100C, which was always from sitting in traffic idling, I would hit the def button for five seconds or so, and the temp would go down to 90 again. But the car on its own never got over 100 on its own, after I swapped out the radiator and condenser.
I replaced the condenser too because even though it look clear, I felt that a new one would allow max air flow through it.
So I would recommeNd that you replace both of those.