manone - let's compare apples to apples here. He's 10s off a time set by Timo Kluck who's a factory driver. Sport Auto's pro driver, Horst von Sauma, only managed 3s better (7:32), which is next to nothing on the 'ring and may be down to ambient conditions.
Well we didn't have those conditions in Chris Harris's tyre test either. The Cup 2 laps were ran the previous day during his Car of the Year competition (all cars on stock tyres), the Trofeo R laps were ran the day after. Mechanics on the scene adjusted pressure in the 458S tyres and the Trofeo R tyres, Cup 2s on GT3 were ran as was.
Not that I even think the difference would be as large as the 3s you suggest but even if we had a few seconds from the comparable lap splits that would still only put the 458S 4s ahead on the Trofeo Rs at Monza, which are supposed to be 2s faster than Cup 2s, yet it was 4s faster even with Cup 2s at Portimao. So either way Chris Harris's tyre test is bollox, as is the 4s 458S to GT3 delta on Cup 2s.
At best I think the 458S is ~1s/minute faster and the Trofeo Rs cut a few tenths per minute. So may a better driver would run 1:58.31 in the 458S on Trofeos and 1:59.31 on Cup 2s. That's about as elastic as the imagination can be though.