In these type of bursty, aggro-combo decks (and specifically an Izzet flavor being my favorite deck to draft/play), I like the extra toughness on
Festival Crasher just to help ward off attacks against other aggressive decks while searching for the pieces I need for that showdown turn. And, I like being able to send in with a 3-toughness vs. 2-toughness creature on earlier turns to chip in a few points of damage to bring the opponent into the red-zone; that extra point of toughness just makes it slightly safer to do so.
That said,
Kiln Fiend has worked really well when the spells aren't so aggressive-focused in their effects. Just playing a bunch of removal, bounce, and card draw on a turn is enough to situate Kiln Fiend as a potent game-ender even without the extra pump spells. Maybe that just makes it a safer option in a cube that doesn't dedicate as much room to those kinds of spells.
Idk, I think the pick maybe comes more down to the supporting cards that will end up in that deck. I find the biggest issue with these decks is getting the last 2 or 3 points of damage, where the combo turn puts the opponent at 2 or 3 life and things just kind of end right there. You end up running out of cards in hand or more likely creatures to threaten with, and most of your top-decks don't look threatening to your opponent after that combo turn. That's why I think strategically, getting in those extra points of damage on earlier turns is so crucial.
Maybe you have given it some consideration or already axed it, but I've found Burning Prophet to play really well in this deck as well. It trades some of the bursty-ness for something I find the deck needs a lot of help with -- digging for more gas! I find more decks will play the Burning Prophet just for the scry effect alone, so maybe you want this card slot to end up wheeling more often into the aggressive players draft pool.