Run HWmonitor.
After a gaming session look at the max cpu temperatures.
The coolant in the aio will take time to heat up and stabilize.
You might see a couple of cores in red recording 100c.
That would indicating throttling.
That is the processor protecting itself.
But if you kept running, that is not all bad if you get an occasional throttle.
look at the clock rates while gaming. They should not be dropping to a low level.
In time, any aio will fail.
The pump, a mechanical device will eventually fail.
It might accumulate debris.
Air will intrude and make the cooler less effective.
I would expect that with a NH-D15s you will be able to run full out.
If you need better cooling than the NH-D15s, you are looking at a 360 or 480 aio cooler.
That may require a new case.
Test with the cpu-Z stress test. I think it represents an instruction mix that is more representative of a gaming workload.
High temperatures will come when all threads are fully busy as in a batch multitasking app.
Not so much in gaming where one or two cores will boost to a very high clock rate.