On the VRM question, not really so long as you have a good case with decent airflow....I have a 8700K overclocked with an AIO on a Gigabyte Z370 Gaming 7 motherboard and have never had issues on VRM temps but I have a decent airflow case in the H500P Mesh.
The 9900K does run hotter but you have a very good VRM setup on both the Extreme and the Formula, so no issues whatsoever...
As to which will provide a better overclock, this really depends on your CPU more than the motherboard as they are both top end motherboards.......I think they both will hit the same max overclock and ultimately it will all come down to your cooling setup as that will be the only limiting factor....
As to the 9900K or 8700K, it just comes down to money...both CPU's are top end and on my 8700K, overclocking is a breeze to 5GHz and a little more if I push vcore....If you are into a lot of productivity workloads then the 9900K if not and more of a gaming rig, the 8700K is also fantastic..Bottom line, if you can afford the 9900K, then that is the best at the moment...