The gain arguably wasn't just a "few %". It was substantial. To the point one could say Ferrari had hands down the fastest car on the more power sensitive circuits. Arguably, they should be leading the championship, but crucial errors (Hockenheim, Monza) and a bit of bad weather (Hungary) have cost them a lot of points in the process.
If it is simply a matter of "turning the engine up", I wouldn't know how it would make sense not to, even at the risk of engine failure, when the choice is between almost dominance [prior to Singapore] to "not fast enough" [post Monza].
Analyzing this "power advantage" Ferrari had, I think it has always been clear that it was especially apparent in qualifying, not during the race. Considering we are talking about one or two straights during 1-2 laps in qualifying vs. 60 laps of 1.5 hour managed and driving to a delta racing that follows on Sunday, I'm not sure how well the argument holds up that they are simply nursing engines to make it to the end of the season and are opting not to use whatever huge advantage they had up to Monza, especially if the gains are made in the ERS deployment area. And as I said; You're best equipped to nurse engines if you can lead the race as that allows you to dictate pace. And considering the performance advantage they did have, sticking the car on pole sounds rather straightforward.
I'd also point out that Ferrari have been using these "power gains" on older Spec-2 engines. I find it doubtful that they are now turning the engines down on fresher Spec-3 engines over reliability and longevity concerns. And they have two cars - if there were reliability concerns, they could still afford to push on one car (Kimi's) to make a difference in the championship.
IMO, it's clear to me that whatever Ferrari was doing and now isn't, has little to do with engine wear, but is either on the electric side that allowed them to deploy more or it is something fuel related (or both).