i would think if the cpu is at stock speed it would be more then enough for the U12A in a push/pull configuration. overclocking it might be slightly different story, i cant say how much though. i found this article while looking for thermal paste comparison chart. they use the i9 9900K overclocked to 5ghz, using the Cryorig R1 Ultimate which looks simulator to me to the NH-D15 heatsink. while look into the heatsink you mentioned i found this video on youtube comparing the D15 and the U12A, worth a quick watch to help make up your mind.
if i were in your shoes i would do a mild overclock on the cpu using the U12A in a push/pull maybe around 4.4-4.8ghz i like to keep my load temps from intel burntest/prime 95 in the mid 60's with it low 70's being my max. it all comes to how loud a setup your willing to have to keep it cool while overclocking. my pc is about 3ft from me so finding that balance is what i do, everyone is different. now in this link is about the thermal past, posted this last since it kind of ties to your AIO question,
thermal paste round up they have the i9900k overclocked to 5ghz so i can give you some idea going from air to water. not all AIO's are going to be the top air heatsinks but if you want something quieter they are worth looking into if you are ok backing your overclock off a bit it your temps get to high. seems since last real overclocking i did with a First Gen i7 max TJ temp has gone up. I have noticed a conscious with the OC community that it running at/around 80c max is safe for long term use with the newer cpus. though we will prolly never have the cpu running that high a temp the entire time we are using the pc, most games dont tax the cpu that much. there are some games that still do, i just have not gone looking for a list. i think that supreme commander is the biggest taxing game for the cpu, i could be wrong.
