I think the thing I'd like to see most here is -1 Zurgo, -1Stormbreath for +2 Rabblemaster and -2 Akroan Hoplite for +2 War-Name Aspirant. Rabbles is too good not to 4-of and with 22 land you can't afford to run many high-drops like Zurgo. Also I feel war-name aspirant is stronger than akroan hoplite. Looks pretty good though.

Soulfire Grand Master not worth it imo, at least not as a 4x, you don't have enough spells to make it relevant. Mardu Charm is only damage 1/3 of the time (more like half of the time because who uses the 3rd option, really?)... Crackling Doom is good but the 2 damage is meh for lifegain, at that point you could just use a seeker, prowess puts him at 3/3 and you heal for 3 (you are usually attacking every turn as Mardu anyway).

Seeker would probably be better honestly.

Ok, so the way I evaluated grandmaster over seeker is always in several ways. Unconditional and conditional. Unconditional you are comparing a 2cc bear vs a 2cc bear with lifelink. Tip of the hat goes to grandmaster in that fight and a fairly big one. Be it offense or defense, grandmaster in that situation is just better. No argument. I always value unconditional awesomeness well over conditional

Conditional is what is being questioned here. There are 19/60 ways to activate prowess in main deck, ~1/3rd chance, 12/60 being an instant speed combat trick. The only 12/60 total ways to activate grandmaster's 2nd and 3rd abilities are the same as the combat trick ability of seeker 1/5th chance. Tip of the hat goes to seeker on ways to activate it.

next comes what is gained prowess vs spells with lifelink. +1/1 per activation. Damage > lifelink as a general rule. Tip Seeker.The 3rd ability on grandmaster is what made up for the push to grand master, it being conditionally very relevant in lategame with the ability to give cycled card advantage if top decked.

The tradeoff after running numbers is is having your opponent loosing 1or2 more damage worth the gaining of 1or2 life as the worst case when comparing lifelink+prowess vs lifelink+spell lifelink. The 7 additional sorcery speed prowess triggers vs the late game buyback ability on the 12 instant speed spells. Which is going to win you more games? If it doesn't work out I'll go back, but I feel it's a really close comparison and nobody can say without extensive testing that one is obviously better than the other. I think the extra life will help push the deck into late game where I feel mardu control and I really want to be.

Is the double black for Hero's Downfall the reason you aren't running it or is it the price? Because this is a solid deck but I feel you need the Downfall's in here for planes walkers and storm breaths mostly. Yes you have Crackling Doom, but that only hits Stormbreath Dragon if it's the largest creature on their battlefield. Mardu Charm and Utter End don't touch it and you don't have many answers for planes walkers. Just a thought! +1!

Also, can you check out my deck Who Needs U/B control!? It's a Mardu Control build I am working on and, despite the name, I have no idea how to combat U/B control with it! Thanks!

I've never been very impressed with Hero's Downfall honestly. I'm more of an utter end person, the exile and ability to hit basically any permanent makes it my general removal spell of choice. The SB stokes, crux, and MB dooms have been enough for me to be able to answer him so far. Kola dragon is also a good play after he drops as he can both block and attack into him freely. If he becomes more of a problem than whip/ascendancy decks are finding themselves to be then I might switch those slots.

The way to combat U/B control is without a doubt aggro. RDW will kill a UB control 9 times out of 10. I'm not quite setup to go full aggro here though, and for it being such a small part of the meta in my area (1-2 guys play it) I find the "going wide" strat to be better than specifically targetting specific builds.

Their weakpoints though are reoccuring things like bloodsoaked champion, ashcloud phoenix, also tokens (do not overextend), planeswalkers, and discard stuff, (thoughtseize, dispise, mardu charm etc) Hopefully that will give you some ideas on how to combat it more effectively. Just remember, they are the control in our matchup without a doubt, we HAVE to be the beatdown.