a well detailed does not 'increase hp'. but attributing to a smoother flow of Aerodynamics
with lesser resistance
eg Start with taking all the junk out of your car's trunk. All that stuff is adding weight. Remember that as little as 20 pounds can change how well a car accelerates & increase 1 bhp.
You can also go for more expensive weight reduction such as replacing body parts with carbon fiber.
coming back to AD , when a car is properly detail, you will have lesser resistance equivalent lesser stress for your engine & lesser frictions
When I drove My 4G63 in my VR4, i did a dyno test, a slight increases of Bhp just from drawing in fresh air instead of eng bay air, heat actually drop your performance. but cool air help the combustion mixtures better, thats where air cool filter helps
same goes to unkept engine, oil leaking & spattering of engine actually holds dirt & heat wouldnt dissipate quickly & bhp is affected
another quick one Air intake - if u were to change to K&N air filter
there will be 3-5 Bhp increase, we did a dyno while i was working for speed works, an increase of 5 times airflows actually ihelp the combustion better.
at the end of the day, its a wholesome thing, if u were to see hot rods, they are built to perform & shine to show...all in all the bigger picture
a well kept car definite will not loose Bhp, but a neglected ones definitely