LEM,
There are several effects going on here. Without the PMC, the N1 will increase with speed but will also decrease with altitude. Just because N1 has increased with speed (ie windmilled) does not mean that it is producing more thrust. (Perhaps this is where EPR was a better measure?) I lay myself open to correction but surely you can’t get extra thrust from windmilling the engine faster by flying faster?
I know my limits and respectfully defer to the more knowledgeable contributors on such performance based subjects.
S&L
Hope the book doesn’t disappoint.