I added cowl flaps to my ES and I think it was worth it.  I reshaped to cowl and front fuselage to make for a more rectangular outlet.  I also reduced the inlet area about 20%, adding a diffuser section.  The outlet area was reduced to about half of the original, depending on what kind of bulges on the bottom are assumed to be "standard."  The cooling is marginal, but adequate - on hot days climb speeds have to be increased and cruise is done with the flaps open.  This is to keep the CHT's below 400 and oil below 220.  Closing the relatively small cowl flaps increases the speed by 2 to 4 knots.  Would I do it again?  Yes, but I think I would figure out a way to increase the cowl flap area.

Gary Casey

ES #157, Lycoming engine