Lets assume your radiator is properly sized to remove any heat generated by the engine.
Then, coolant flowrate is determined by the orifice size of the thermostat. I run a high flow thermostat,
and the engine stays under 200 most all the time. If I ran a smaller orifice size thermostat, that restricted
flowrate, then the engine would run hotter. The smaller the thermostat flowrate, the hotter the engine would run.
So, seems the only need for a restrictor plate would be to make your engine run hotter, if the thermostat
was allowing too much flow. But all you would have to do is buy a thermostat that allowed less flow.
Or, it seems, a restrictor plate would also help maintain engine temp, if the radiator was oversized, and did too much
cooling. Rarely do we see that.
But again, restricting coolant flow can have bad consequences in the head, especially at 6000 rpm.