That is just wrong. The fuel flow restriction was added to reduce engine power output. The sliding scale stops at 10,500 RPM. After that you are not allowed any higher fuel flow for any higher RPM. This pretty much obliterates any reason to rev any higher.
The flow restriction was introduced tp cap power. The sliding scale was added to make sure that they used more rpm.
The reason to rev higher than 10,500rpm is that there is more power above that than below 10,500rpm with the reduced fuel flow.
That doesn't make any sense at all. Stop listening to whoever told you that.
If the fuel flow restriction was removed it is likely that they would use less rpm.
No sense to you, perhaps.
The fact is that if the fuel flow restriction was flat they would use the least rpm possible. This is because higher rpm causes higher friction losses and thus less power. The RPM would be set by the air flow required to use the full fuel flow. I have no doubt that with unrestricted boost (as these engines have) that will be at an engine speed lower than 10,500rpm.
In the early 80's the engines were limited by mechanical valve springs and 12,000 RPM was indeed high RPM. It wasn't like they had any choice to go higher. Pneumatic valves started to show up in 1986 and in a few years became standard and with them came higher RPM.
They were also, from 1985, boost limited. If you can't stuff more air in with extra boost then you need to rev higher.
The Honda RA186E engine of 1988 still only had a maximum speed of 13,000rpm.