Now you're onto something.
A few thoughts,
1) I'm going to wager that Magma Jet will be better than Shock 9/10 times. BUT, I would only run 3 of them and then two Lightning Strikes (aka Searing Spear/Incinerate). This lets you burn for two, scry, or burn for three on larger targets.
2) Purphoros is going to be big in these decks. Being able to land him on T4 will almost always seal the deal, if for no other reason, than because he will likely finish the opponent off in the ensuing turns with his creature ETB damage.
3) Another four-drop to consider would be Fanatic of Mogis, who, if it comes into play after you've been dropping creatures, can easily do 4-5 damage immediately upon entering the battlefield - potentially enough to finish them off just like Hellrider used to.
4) Everyone I've talked to/heard of running Chandra raves about what she can do. I would suggest upping her to 2x. It'll provide you with some card advantage and might help force those final points of damage through.
5) Foundry Street Denizen and Rakdos Cackler should both total at least 3x of each, if not 4x of each. Remember that this type of deck will likely come up short if you don't curve out pretty well on a consistent basis.
6) I would suggest upping your land count to 21 or 22. The 20 land count SHOULD get you there most games, but adding 1-2 more will just ensure things are that much more consistent.
- For the sideboard, Mutavault doesn't make sense. You'll either want those land all the time or not. Additionally, Flames of the Firebrand really isn't ideal. You should focus more on getting the damage through. Perhaps something like Seismic Stomp will work there... although there are no guarantees. I saw your note that you intend to add Akroan Crusader and Coordinated Assault, although I would suggest against both of those - the Crusader needs you to target HIM, which you're not going to be doing with anything and Coordinated Assault is just a lesser Weapon Surge. Perhaps you slide Weapon Surge into the sideboard to help you blow the opponent out in a mirror match, but otherwise, it's not enough damage to really make the difference. If you're trying to push more damage though, Dynacharge will manage that more effectively.