Vs. Aggro = Comfortable in the control role.

I know what I want to do against aggro.

I remove their early threats.  Get card advantage with my 2:1 burn spells and Treasure Hunt.  Go 1:1, or get mana/quality advantage with my filter/cheap spells.  Eventually land a PA, and burn them out.

Recurring 2:1's with Call to Mind, preventing big swings with ITR, countering hard to deal with spells with Deprive (and potentially MBT post-board).

I feel very comfortable in the control role here.  There's no way for me to out-aggro Memnite, Goblin Guide, Lacerator, etc, and I have good answers to them as part of my main plan.

Maindeck/Sideboard consequences:

Playing Arc Trail main (which seems as good or better than Burst Lightning in any aggro matchup) means I don't need any sideboard slots for this matchup.  The Arc Trails are the first thing to come out vs control/ramp, so having a "more generalized" Arc Trail maindeck, and siding into Arc Trail, or siding Arc Trail into Pyroclasm for aggro matchups seems like a waste of 4 slots.

So Pyroclasm really feels like it can/should be benched.  It is too reactive, and is no more of a 2:1 than Staggershock or Arc Trail.  The scenarios where it is a full on blowout are few and far between, and it can never go to the head.

There may be some argument for putting the new 3-damage Pyroclasm (Slagstorm) in the board, especially if the green leyline, or more token-based strategies become prevalent.  The 3 damage vs 2 is not terribly relevant, and the 1RR vs 1R casting cost does not favor Slagstorm very well.  However, the 3 damage to the head option makes it look pretty good main deck.  It also doubles to kill titans, and is a non-targetted way to kill some bigger things (especially doubled), like the Trool, Gaea's Revenge, Sphinx, Frost Titan, and so on.  But we'll talk about that later.

So, in general I feel pretty good about the aggro matchup.  They can put incredible pressure on early, but I can't think of a single deck where they have no top-end threat, or no recursive threat, or where ITR is bad, where I would be willing to take out ITR + counterspell, for 4 creature sweepers.  So there is no room to play all the red spells, so the only thing to do with sideboard slots for this matchup is to upgrade existing creature kill with better creature kill.

/shrugs to that.