Stormchaser, along with Swiftspear and Abbot of Keral Keep, means that you can play 12 creatures that cost 2 mana or less that are all capable of dealing lethal damage on their own, with just a little help from some spells.
Temur Battle Rage is exactly what this deck needs. Just a couple spells and prowess triggers are more than enough to make Temur Battle Rage lethal, even in the face of potential blockers. I would absolutely want to play 4 as it’s the card I most want to draw when I already have a prowess threat in play.
Slip Through Space is an awesome addition from Oath of the Gatewatch. This card seems simple, but it does so much. Modern Infect decks have been seen picking up some of these and it’s much stronger here. Slip is already a 1-mana cantrip, which is great in a prowess deck, locking in at least a few damage. In conjunction with the best pump spell a UR deck can muster, Titan’s Strength, you are looking at 6 or more damage at a minimum.
This is also one of the best Treasure Cruise decks. All of the threats are super cheap, the cards that trigger prowess often replace themselves or get a scry, and you can play plenty of fetchlands. Leading with a 1-mana Cruise on your turn often means that you can play a hasted prowess creature and start pumping it, already adding more fuel for the next Cruise.
I like Painful Truths in this deck in particular, as you’re applying tons of pressure, so the life loss is less likely to matter, and it’s great at refueling with more cheap spells.
This is one of the most explosive and fun decks I’ve seen in Standard and it may also be strong enough to have serious lasting potential!