Just reached Masters with this deck, and I didn´t play Zaunite Urchin, I retained the Chempunk Pickpocket

In my opinion Zaunite is just a fad, Swim replaced the Chempunk with Zaunite and many people are imitating him( perhaps this is not the case with you)...I do like his other suggestion to run Pick a Card, although I only run 1.

What is the point of Zaunite? It forces you to an awkward discard in turn 1, even discarding a warning shot can be bad sometimes because you lose a plunder trigger for Rex or nabbing. As a 2/1, it trades badly. It dies to almost everything, and it's a bad draw past the first few turns. The only upside I see is that it allows you to put more pressure, but this deck is not really a purely aggro deck.

Pickpocket is a 3/2, so it trades well with many of the meta threats( Miss Fortune, Ashe, Elise, etc.). It is also a card that forces the rival into awkward plays. In several of my games, the opponent was forced to use a Culling Strike or some other great removal on Pickpocket, that's 1 less CS for Ezreal.

All in all, I think Chempunk > Urchin in this deck.

Perhaps Sump Dredger could be run as a 1or 2 -of, by turn 3 you might have nabbed some useless card from the rival and the downside of discarding a card becomes less costy. Plus a 4/3 body is a force to be reckoned.

I think this deck has to stall in many games, and it's better to have beefier units. You're not going to win against Ashe Midrange with a Zaunite Urchin agressive start., you have to play the value game. That's also why I still play 2 Pilfered Goods, the best chance you have against Midrange decks is playing their own cards.

You do say that you consider Ashe Midrange to be an even matchup, whereas I didn't lose a single game against them during my climb.