Finally decided to see this for myself, it's very true indeed.
I was doing the Super GT championship (GT500) and my car of choice was the HSV. To make things more fun, I decided to run it on Sports Soft tyres. Now, throughout the races, I already noticed that the first 8 AI cars were on the same pace than me, often even making the gap from 1st become bigger, but on lap 3/4/5 they started to slow down a lot, allowing me to close the gap and win. On the last round at Nurburgring I checked the replay. My best time on my SS tyres was around 1:29'1. The lead car on RH and some others in the top 8 had done their best times that were faster than mine by about half a second. But around lap 3, they all decided to lap around 4 seconds slower, again giving me the win.
That's poor, especially now that I realize the AI isn't that slow on it's own, it's the rubberbanding that destroys most of the experience.