That's strange - I had some of the same symptoms and it ended up being a completely different thing - a dying (and then eventually completely dead) SSD (the Phison controller fiasco).

Other than that the CPU itself seems fine and it's only shown very minor degradation over the 4-5 years I've had it - i.e. it used to boost up to 4975 or 4950 single core, now it only goes up to about 4900 (no PBO, no CO); and I've also had to fix the SoC voltage because of the memory - but I haven't touched that in like 3 years, so not sure it's even degradation.