@Defense Grid: I used to play 3 Defense Grids during the mental misstep area (mostly because they didn't cost 1 ) and it was definitely fine. They basicaly can't do anything in their own turn they want to keep force mana up which give you plenty of time to set up your hand. It is however obviously way better with chants (but I haven't played white since they printed Abrupt Decay ).

I'm still unsure is Xantid Swarm isn't better in comparison to Defense Grid. Defense Grid would be sided in in more matchups and doesn't die to bolts or stp but has the downside of costing one more mana and having weird interaction with discard spells...

@the combo matchups: right now I play flusterstorm to combat the mirror, Show and Tell and (to a lesser extend) Reanimator. But I'm not at all convinced they are worth the slots since Show and Tell decks aren't played as much anymore and Reanimator doesn't improve that much with flusterstorm.
Which there was a card that would solve the reanimator and storm mirror match but I've jet to find one unfortunatly.