Take a card out of sideboard, idk what to suggest so it is up to you

I'd drop the mizzium mortar and add Lightning Strike . I do not know what you would drop but you need to get some Ghor-Clan Rampager because Stormbreath+Ghor Clan Bloodrush+ Blood= 16 damage.Also I would add Plummet Skullcrack Destructive Revelry and possibly Savage Summoning .

Straight Monstrous?
As opposed to Gay Monstrous? :D

I would cut down on the pump spells. You only run 16 guys, and sometimes you run into the infect problem (all pumps, no guys). I'd consider a small number of Magma Jet and possibly Dig Through Time to make your draws more consistent and continue to provide you with gas.

You should really be playing Sarkhan, the Dragonspeaker . I'd love to hear why you aren't, though.

Sarkhan is overpriced. I'm trading mine for 2 Mantis Riders, a Purphoros, a Nylea and a Chandra. I can bring in 1 spare Dig. The reason I'm hesitant to is because the gas (card draw, as it were) is being given by the Flamespeaker. Do you think I should go all burn suite?

Going for full-on burn sounds solid given the fact that Prophetic Flamespeaker requires you to cast the cards the turn they are exiled. Cutting the Banishing Light for Magma Jet to support that, as well as adding the all-important Lightning Strike sounds like a very cool plan. Why pump your guys when you can just burn their guys off the battlefield?

I can address that. If you have a big enough rear on the battlefield, my Flamespeaker won't have a chance to exile a card from the top of the library, ergo can't burn them out.

Maybe Dictate of Heliod ?

I would cut the Swiftspears for Lightning Strikes. You're too creature-heavy for the Swiftspears to be efficient.

Do one of two things. 1.) Cut Monastery Swiftspear for Lightning Strike . The man abase is too unreliable to consistently get turn 1 red into jeskai turn 3. There's a reason jeskai wins doesn't play it. 2.) Cut blue. Your mana becomes more consistent, you can now run Chained to the Rocks , and if you do this i'd also probably add in Lightning Strike . Another suggestion for this build would be Dragon Mantle . This would let you lower your curve significantly, and mean you could play less lands. (I play RW tempo and i run 21 or 22 (forgot which)).
tl;dr swiftspear is bad in jeskai, but really good in RW. Pick between blue or swiftspear.

You need strikes, the Jeskai/aggro games are super rough without them.

Id drop the Titan's Strength for Strikes, the effect is basically the same but more flexible being able to kill blockers pre combat or just go to the face for the win on a gumed up board. And I think the flamespeaker is activly bad. Id play Brimaz or Ashcloud's over them.

I'd drop Titan's Strength for Jeskai Ascendancy for the Stoke the Flames synergy. Maybe sideboard Gods Willing and put in Lightning Strike . I think it would kill a lot faster. Jeskai Ascendancy even buffs Prophetic Flamespeaker .

Howl of the Horde would also be hilarious.