A buddy of mine and I came up with this list after seeing the new Chandra spoiled. Being so splashable, we decided that she belonged in a Superfriends type deck. After going through Grixis and American Superfriends, we decided that straight Red White offered the most potential and the most flexibility.
I'm still advocating wur super friends. The interaction of Venser and the new white dignitary is to good. Also blue gives you mana leak if you want to go with it. And of course access to draw spell and bounce. I like your list for what it is though I see no reason to include karn. I just don't see him as that playable. I'd rather have inferno titan really. Pyroclasm should be sideboard at best. Unless your meta is filled with weenie decks there is no reason to main deck it.
More pressingly, I can understand running Arid Mesa here...but...4 Terramorphic Expanse? I fail to see how this deck needs the landfall, and even if it did, I'd personally rather just go with Marsh Flats and Scalding Tarns, even though it limits my color choices.
Yeehaw...minor deck thinning that slows me down?
Also, isn't 26 lands a bit much anyway? Or am I severely under-educated in regards to the superfriends archetype?
Your Chandra is useless. She can copy Bolts and Bursts and Gambits... Thats honestly not a lot of value. You should try and put some more powerful instants/sorceries in there for more value. Deffinatly cut Burst for Volt Charge.