I know Willy Edel said Pharika's Cure underperformed so I may want to replace that but I also don't quite understand some of the other SB choices.
A lot of it is really going to depends on your local meta and what matchups you face. Pharika's Cure is really amazing against mono red rush (basically kills any of their creatures and gives you 2 life) and it's really strong against Master of Waves. I found myself sideboarding in multiple copies last night. I think the card is great in the current meta, but if he faced a lot of RG Monsters, then yeah I can see how the card doesn't do what he needs and Doom Blade would be way better.
Erebos/Underworld would come in against control to gain more card advantage.
Gaze of Granite/Doom Blade/Pharika's Cure/Ratchet Bomb/Vraska/Devour Flesh would come in where you need more removal. Vraska could also be an alternate win condition against control I guess.
Golgari Charm to fight Hammer/Bident/Whip
Pithing Needles is probably to deal with Planeswalkers
The Specters are the only interesting thing to me. There would be matchups where you want more threats or more flying. Against Mono Blue Devotion these would be great. I don't know if he was expecting to see a lot of that deck or what else he would be planning for.
edit: Derp, it just hit me that these come in against control to replace the irrelevant removal spells. It just ironically was also really good against Mono Blue Specters and Raptors.
I don't really like so many 1 ofs in the SB, especially something like Doom Blade/Devour Flesh that do very similar things, but of course are also very specific. Devour Flesh beats hexproof (Caryatid, Witchstalker, etc.) and Doom Blade beats everything else (that's not black obv.). I would rather see a 4th Hero's Downfall than either of these. The extra mana cost can be a consideration, but the flexibility is much stronger.
I'm obviously not a pro and really just getting back into the game. I've been watching a lot of SCG and WotC streams of their major events, so I'm still trying to understand the meta myself. The States tournament on Sunday saw a lot of GW aggro and mono red still so all my removal was good. Last night at the smaller local tournament the PT had an effect and saw more mono blue, mono black and mono red devotion. If you know what you'll be seeing it should help you to shape your SB a little more to handle it. Also keep in mind FNM is not the same as the PT, not just in level of play, but also in size. 5 rounds of swiss is very different than 10. You'll face a less diverse group of decks; you'll need less silver bullets like this.
To me it seems like a solid deck, but I don't know that the green adds enough (Reaper, Scooze, Abrupt Decay, G Charm, Gaze) to make up for the inconsistency you could have from playing Mono Black. I'm interested to hear how it does though and more importantly if it's more fun...it's always the best choice!