As stated before, now that there is no Mental Misstep in the metagame, I think the OP deck is the better starting point instead of the one from Chris Renner. It (the OP deck) has arguably 2 flex slots, and so I wanted to discuss some options:
I often think Daze and Force aren't enough counterspells. But I don't like Spell Pierce nor Spell Snare, as those aren't broad enough to warrant MD action. Stifle I can get behind, as it can add to the mana denial strategy or do some fancy tricks. Anyway, I believe the best cards are blue creatures - they are enabled by Vial, pitch to FoW and can carry Equipment.
Phantasmal Image seems good in those flex slots, as it can legend-kill both Progenitus and Emrakul, and can be Vialed in to fight Iona and other Legends as well. It is better than affording that one Karakas just to have it Wasted more than to get it used (Karakas feels like a nonbasic Plains far too often). And (obviously =P) it can copy whatever the best creature on the battlefield is.
Delver of Secrets is just another beater. I'd test it, but I think there are fancier options. Besides, this deck does not have many ways to ensure it becomes 3/2 reliably, as usually it has 4-6 slots dedicated to Equipment and Vials, and the cards other than FoW, StP and Daze usually are either lands or creatures.
Snapcaster looks to be subpar in this deck. Lack of targets to Flashback other than Swords and Brainstorm make me think it is not that good here.
Spellstutter is one of my favorite Equipment-carriers, because it adds some critical mass for the counterspells as well (as stated, I feel FoW and Daze alone aren't enough sometimes) and isn't reliant on other Instants in the deck.
But Meddling Mage is better than Sprite when one knows what to name: Ad Nauseam, Show and Tell and Life from the Loam are some of my most-often named. Unfortunately, it is more of a SB card.
As I had a hard time deciding between Image or Stutter, I chose to run both and move the Grunts to SB, alongside the Meddling Mages. It gave me 10-12 counterspells (sometimes I cut 1-2 Seers, or 1-2 Moms. I like having the full set of those though), more cards to pitch to FoW and a cheap, MD answer to Progenitus and Emrakul. On the other hand I lost Grunt and whatever hopes I had of fighting Dredge or Loam recursion Game 1. I try to compensate that by dedicating the most SB slots to graveyard hate. If those, alongside the rise of Snapcaster-enabled strategies, become more popular, I think I will favor Phantasmal Image over Sprite, and run Grunt MD.