Soldato
The only way you are going to beat a top end air cooler such as the D15 is to go custom. AIOs are very cleverly marketed and people get suckered in all the time, but there is no real benefit other than the aesthetic. I totally get that though, and having a big hunk of cooler (and they are BIG) hanging off your mobo isn't what everyone wants. It dominates visually and if you're moving your system around a lot, I can see the concern... not that I've ever heard of one causing damage. You hear about AIOs leaking, so that has to be factored in. Custom loops aren't cheap, but a CPU only one is much cheaper (probably a £200 saving or thereabouts), and will offer better reliability and the ultimate in cooling and low noise performance... if done right of course. Yes it costs, but the best always does.
Ultimately it all depends on ventilation of the case, the cooler is only one part of the equation, you can have a £150 AIO in a rubbish case with no ventilation and airflow which is bested by a £35 air cooled heatsink in a good case with sorted airflow. Mine pictured above has three 120mm low speed exhaust fans above and behind it and two 140 intakes at the case front.