I support the nom for Mega-Beedrill to be placed at C- or C
On the the surface, the mega speed buff is indeed a big thing for Mbee; not running protect means it can run SD+3 attacks or AoA now (most notably being U-turn/Pjab/Knock off/Drill Run). However, upon a quick analysis, one will see that it didn't really got better in SM, most notably due to the introduction of Pheromosa, which highly outclassed it, as well as the Landorus-t, which walls it, being more popular as ever.
It's niche over Pheromosa is just being able to OHKO Fini after rocks, and living pre-Ash-Greninja's Water Shuriken (which 32 sdef Phero does, assuming 3 hits). Meanwhile, it is rocks weak (which highly ofsets Pheromosa getting worn down from Life Orb recoil), it cannot use an item, uses up your mega slot, which is a big factor considering how strong the likes of Metagross-Mega is at the moment, and most especially, it is
walled by the most used mon in the tier, Landorus-Therian.
Furthermore, it is only extremely slightly bulkier than Pheromosa on the physical side, and is decently bulkier on the special side; this means that it also dies to the priority attacks that Pheromosa drops to, which are: Mega Pinsir's Quick Attack, Adamant Choice Banded Dragonite's Extremespeed, Adamant Choice Banded Zygarde's Extremespeed, and Choice Banded Crawdaunt's Aqua Jet.
Meanwhile, Pheromosa can OHKO the other Tapus which Mbee is touted to do with its Adaptability Poison Jab. For example:
252 Atk Life Orb Pheromosa Poison Jab vs. 0 HP / 0 Def Tapu Koko: 270-320 (96 - 113.8%) -- 81.3% chance to OHKO
252 Atk Life Orb Pheromosa Poison Jab vs. 0 HP / 4 Def Tapu Lele: 299-354 (106.4 - 125.9%) -- guaranteed OHKO
252 Atk Pheromosa Poison Jab vs. 0 HP / 4 Def Tapu Bulu: 320-380 (113.8 - 135.2%) -- guaranteed OHKO
Pheromosa's Choice Specs set also OHKO Tapu Bulu with Bug Buzz, as well as Tapu Lele after Rocks.
As an added liability, it doesn't live Mega-Mawile's Sucker Punch, while Phero lives it and proceeds to OHKO afterwards with a Z-Focus Blast, or OHKO with regular Focus Blast at +1 (after a Quiver Dance Boost).
Thus; in the end, while it is touted as a fairy killer, Pheromosa just does the same, without the massive costs that comes with using Mega-Beedrill. In the end, its position didn't really got better since ORAS, and it should remain at the lower ends of the VR.