The early motors, in particular the 2.5L, suffered from a lot of teething issues, including hot spots in the cooling loop caused by poor circulation and air entrapment. From what I have heard, the line from the oil pump was a circulation and air entrapment bandaid that was "fixed" by altering the case castings to improve flow on later engines. If that is correct, I would not cap off the oil pump line on any engine that has it connected from the factory as you may end up somewhere you did not intend to go.
If you ever get the chance and you want to do an interesting comparison sometime, look at both the early engine oil cooler without the top water connection and the oil cooler on the Tiptronic transmission; they are very similar in layout, but I have not had the chance to see if they are interchangable.....
