Outside of the Treasure Cruise era, U/R Delver has been relegated to the fringes of Legacy, the awkward and underpowered cousin of its Temur and Grixis brethren. The deck has always felt like an odd compromise between a Burn deck and a Delver deck while always being the worst of the three.
Monastery Swiftspear has helped to bridge the gap by allowing you to convert your blue cantrips into damage, and Stormchaser Mage may be the last piece of the puzzle in that regard. Once you have multiple prowess creatures on the battlefield, every Ponder and Gitaxian Probe is worth nearly as much as a Lightning Bolt, and because they replace themselves, you are almost assured of never running out of gas.
The other draw to U/R is Price of Progress. Without a third color, you can play enough basic lands to not be bothered by the symmetry of this powerful burn spell, while dealing six or more damage with ease. This makes U/R by far the most explosive Delver variant available, without sacrificing much in terms of your disruption package.
The downside of having more prowess creatures is that your counterspells become more awkward. For that reason, I would like to see the Spell Pierces become something proactive, perhaps another cantrip like Mishra's Bauble, another burn spell like Chain Lightning, or a way to fight bigger creatures like Vapor Snag. With the added speed, I think Force of Will and Daze should be enough to race the combo decks before sideboarding, and being unable to consistently pump you prowess creatures is unacceptable.
Despite losing a color, this deck still sports an impressive array of sideboard cards, all of which we have been seen regularly in sideboards of three-color Delver lists. All except one, at least. Blood Moon is a great way to surprise midrange decks like Shardless Sultai, not to mention Lands, as it is not a card anyone typically associates with Delver decks.
Maybe I'm just not ready to give up Stormchaser Mage after our recent love affair in Standard, but there is a lot to like here. This deck could surprise some people in Philadelphia, as players with fewer opportunities to play Legacy Opens roll back in their testing of the format and ignore the fringes in favor of metagaming against more established decks.