This question raises several issues around the development of cooling systems on WW2 German aircraft. The problem is that there is considerable drag associated with the required area of the radiators, so the basic idea that inline engines are intrinsically less draggy than radials is shaky at best. The annual radiator or RingKuhler immediately behind the propeller (I think that’s the correct German term) as on the Ju88A was an attempt to reduce the overall drag of the entire system at the expense of frontal area. This did make it more convenient to exchange complete power units (“the “power egg” concept”) and also greatly eased the conversion to radial engines, as on the BMW801/JU88R. As power increased so did the cooling requirements, and radiators had to become even larger. The final German approach was the Drumkuhler, where the front opening led to a ducting so that the actual radiators were not face-on to the direction of travel but parallel, reducing the overall frontal area and hence drag. These could be seen on the Fw190D, Ta152 and He219, amongst others. The final designs of German piston-engine fighters including the Me209 adopted this system, whereas the Allies went towards greater us of ducts to large radiators inside the fuselage or wing – most notably the P-51.