" . . . not gain much . . " That's probably true, but you won't match D15 performance to, say, an H100 or H110 unless you "do certain things," likely "spend some extra money on fans," or set aside a day for Xacto-knife tedium of the duct-building kind. Either or any way, you're going to spend $10 or materials and adhesive. You might spend the same for a $5 item for a total $10 with shipping. You may likely have to re-evaluate your D15 fans, although I understand they're better than those bundled with the D14. You might still want to spend $20+/- for a rear-exhaust fan replacement. If your objective is to kill as much noise as heat, add another $8 + shipping for some foam-rubber acoustic pads with adhesive.
I just say this because I myself have put off any investment in water-cooling by investing labor and time in making air-coolers "better." That also includes lapping the processor and heatpipe-cooler base to bare copper, and spending a few more dollars on diamond paste, Indigo Xtreme or Coollaboratory Liquid Pro. The temperature improvements are all additive.
I'd be more inclined to use an AiO for a 4790K, but I might attempt using either my D15 or ACX cooler to see if I can get either to perform the same way. Intelligence gathered here at the forums is encouraging. I think I could probably cool a 4790K to ~72C (again . . +/-) under burn-test maximum loads, at the same time overclocking to 4.6 -- maybe 4.7. Lucky? 4.8.
But I'd have to be building a Devils Canyon rig to prove it firsthand. And I have a budget and a schedule (which keeps slipping), while leaning toward an "E-" system.