Yes, a higher temp will yield a better delta T, but with that logic, you have to look at the other side: if it's too hot, then the water can't pick up the heat efficiently, and you end up with higher temps.
This is where it becomes counter intuitive, because you would normally figure that by slowing down the flow, the water would be able to absorb the heat, when the reality is that it will actually make things worse, because your coolant will heat up, and your baseplate is still very hot. (or something like that...)
The solution is to decrease the resistance of the heat transfer, to the water. You can do that in a number of ways, classified into two categories:
1- make the flow turbulent
and/or
2- Add fins/pins
#1 is extremely hard to achieve, because the heat transfer is exponentially proportional to the flow, in an unfavorable way: i.e. double the flow rate, will lower the resistance by 1/4 (a very rough approximation, meant as an example).
Optionally, one can use jets, or small orifices to project the water at a higher speed, against the baseplate, which creates a turbulent area, right where you need it.
#2: pins and/or fins have the effect of spreading the heat upwards, keeping the baseplate temp a smidge lower. Don't dismiss the scores/grooves, because that's actually a fin, in reverse.
There are a number of references available on all this.
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