I tried with my pump, which is much like a Eheim or any other magdrive. The impeller housing (where water flows) is pretty well insulated from the engine block. Actually there is no connection - hence the 'mag drive'.
The engine has its own housing and on good pumps such as Eheims this housing has fins - air cooled, baby.
So an inline pump wont add noticeable heat to the water. However if it's inside your case it will heat up the air quite a bit indeed ! (this is very noticeable with more powerful pumps)
Now, before or after the rad, i think if your flow is high enough it doesnt matter.
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