Forget "better" charge aircoolers, if you want better efficiency buy a efficiency chip tune. And from a tuner who is specielized in efficiency. -10% is easy which most ecu tuners can reach, but there is usually even more available.
If the size is same very little room for improvement of efficiency, smaller fins help little. You migth get few more horsepower, but for fuel efficiency impact under 1%.
My best in my 4wd 2.5 tdi touareg has been 5,4l/100km at 100 km/h speed. Some ideas here. Whole underside is flat in that, but no pics from that.
https://www.facebook.com/media/set/?...82242155122992