Slagstorm or Pyroclasm would definitely be solid sideboard cards. And Beast Within is starting to look better and better. =)
I think Plated Geopede and Grim Lavamancer might be a little underwhelming when you're only running 2 Fetchlands. And the Birds of Paradise acceleration seems weak with that deck - no 3-drops and only three 4-drops. Four Swords might be a bit too many.
R/G "New Fires"
To reiterate, the plan with my "New Fires" tempo deck is to skip the 2CC slot completely and ramp up to 3CC and 4CC a turn early. Phoenix and Vengevine are hasty beaters that CAN come back, but the ability to return to play is not why they're being used here. Vines of Vastwood gives your creatures some protection from removal anyway, and burn spells and Beast Within clear the way for them.
Filling the last 2 spots and making the following changes:
-1 Tectonic Edge
-1 Vines of Vastwood
-2 Red Sun's Zenith
+3 Beast Within
I actually really like the idea of RG haste aggro.
It's got a lot of good options in the meta right now. I can see it being good vs. Control with Chandra's Phoenix & Vengevine, and the rest is simply a matter of racing the combo decks while being able to play the control deck effectively vs. RDW.
Urabrask is also an effective "answer" to splinter twin too, which is nice.
I've come to realize that W&P is the better sword for this deck. And I'm really trying to push for strong match-ups against RDW, Tempered Steel, Vampires, and other random aggro decks, especially after sideboarding.