Drag not so yet as we only stress the engine in couple of secs only but track is totally diff. If you notice, most of the track car or those who often go for track do have oil cooler installed. Track session is like continuosly stressing your engine for mins. Imagine a lap of PG you clock is under how many mins not to mention Sepang track. So you can roughly know how long you stress your engine continously. So oil cooler is crucial.
some of the Drifters also do installed oil cooler due to the car is always on high sideway, the air will not direct hit the radiator hence engine temp can shoot up easily. not to mentioned on drift, you're on constant high band reving.
there is 2 general thumb of rules for selecting and installing oil cooler. Again, Bigger doesnt means better. too big will result pressure drop same goes to installing oil cooler too far frm the oil supply jack. Again, engine oil need a certain temp level to perform the best. Thats explained why always warm up the engine before heavy whacking