I guess it comes to Gamble getting anything and Tutor getting, hmm.. Loam and Gamble + possible 1-ofs? Tolaria West is quite mana intensive and absolutely needs heavier commitment on blue. I want to use my mana for activating manlands and using Ports. 3cc sorcery-speed tutor quite doesn't sound too appealing when you compare it to Gamble.
Mystical Tutor in my opinion also contributes to making the deck vulnerable to Chalice of The Void. Basically all the business cards besides loam we want to see in opening hand are shut down by Chalice for 1 and that is the reason I also want to advocate playing Burning Wish or Intuition, but I'm not experienced with the blue splash so I leave that to others.
About Upheaval being difficult to cast: I was referring to the need of double blue, which is two blue mana more than this (moxless) GRW-build has access to. It might work for the blue version, but IMO that is just a different deck.
I usually side in some form of graveyard hate when playing against enchantress to nullify Replenish. Bojuka Bog is the newest toy in lands' arsenal. Karmic Justice is bollocky, but if they are in postition to nuke your whole board due to your sweeper, you were not going to win that game anyway. After all, you are the premiere permanent deck. I'm more afraid of Sterling Grove, as it makes removal of Wheel of Sun and Moon, KJ or any other obstacle on your way a pain between your buttocks.

