Someone here runs something similar and has done fairly well; some FNMs top 4, some not, just depends on what he ends up playing against.I'd honestly suggest just running more basic lands; you don't need Shimmering Grotto and you don't REALLY need Nephalia Drownyard. Take out the Mountains, and run a bunch of Island/Swamps (probably Swamp heavy), and perhaps switch to Dragonskull Summit instead of Blackcleave Cliffs.
The reason for the specific choices above is that you currently have 3 cards in the entire deck, not counting sideboard, that require red, and all three require only one red. (Nicol Bolas & Devil's Play.) If/once you switch to Bonfires you won't need RRR to flashback Devil's Play, which you're kind of unlikely to get even with your current manabase anyways. Dragonskulls over Blackcleaves because you'll have more swamps/black sources. You don't need the red mana early on, but you probably do want it to come into play untapped later on (especially if it's the 8th mana you needed for Bolas).
Consider Chandra as a 1-2 of, for her -2 ability. It's not great, but something to consider still.
Consider Jace, Memory Adept over the Drownyards as an alternate win condition.
If you go the more-basic-land route, consider Solemn Simulacrum over Sphere of the Suns. Also, I don't know if you'll need the Gilded Lotuses. If you take out the 2 Spheres & 2 Lotuses for 4 Solemns, you should always reach the 7-8 mana for Karn/Bolas. Perhaps use 1 Mountain if you go Solemns, so you can always search for the one red source you'll need.
The guy here runs Sever the Bloodlines as one of his semi-mass removal/targeted removal. It works fairly well, and prevents a lot of "when x is put into the graveyard" or "when x dies" abilities because it just exiles. And it has flashback.
You could probably benefit from some extra reach somewhere beyond just Ponders. Amass the Components is a fair choice, so is Forbidden Alchemy (though not terribly awesome) or just simply Think Twice or Sign in Blood.
Negates/Mana Leaks are kind of up to you, but I think Negates might be a better choice here; Mana Leaks are for early threats but honestly you shouldn't have problems with creatures with all the removal. I don't know about Dissipates, you might have to playtest with that a little bit. The guy here said he wishes he could run them, but often runs into issues getting the UU needed for it.
[Edited 3 times, lastly by Bagbokk on August 25, 2012]