They have to use the same 8 ratios for Monza as they do for Monaco.
So, if they can use 8th gear in Monaco it will be low in the rev range - well below 10,000rpm. At Monza they will be pulling 12,000rpm in the same gear, or more.
I doubt anyone actually used 8th at Monaco, and rarely atthe other circuits so far.
In previous years they had one less gera, 7 in total. But they could change the ratios and/or the final drive to suit each circuit. This year they are stuck with what they've got.
Yes, you are right. I was thinking they could still change ratios when ever they wanted. Now they can only change the set of ratios once per season.
I suspect they simply do not use 8th gear at the slow tracks. But they still have a gear for each speed range. They would not drive around at low RPM when they have a lower gear if they were racing, they might do it to save fuel though. So I don't see where that implies any thing about the power band really.