White's the best splash color, but it's not leaps ahead. No one is building a deck and adding White solely for 1-4x Copies of sideboard cards*
As for Jeskai being a deck, it had one Flash in the pan over the past 2 years during the Nahiri Hype and really hasn't made any improvements since.
*The only deck that runs a white splash solely in the side is Elves, and that's because each White Hatebear has effectively 9 copies for each 1of(1 of, Chord, CoCo.)
Actually, it's definitely been done before. Maybe not in the current meta.
I was speaking of UWR being a deck actually before Nahiri. It's been awhile. Won the Pro Tour in 2014, I believe. It was fun to watch Shaun McLaren win that one.
Ryoichi Tamada got top 8 in Kobe by going back to basics with his UWR Control a couple of weeks ago and the deck has been posting tons of 5-0s since. Could have legs!
Saw some tech on using Ulvenwald Hydra to grab Shrine of the Forsaken Gods to ramp into Ulamogs from hand. Nice. I'll run that.
Brad Nelson was running that last week when he won GP Omaha undefeated. He said it was the best way to fight the decks that get rid of your Marvels (so you can do your thing without relying on Marvel g2/3).