M-Beedrill is one of the reasons I joined Showdown and started to mess around. I run this set, which is quite similar to the OP:
Beedrill-Mega @ Beedrillite
Ability: Sniper -> Adaptability
EVs: 252 Atk / 40 Def / 216 Spe
Jolly Nature
- U-turn
- Drill Run
- Poison Jab
- Protect
Now, it may seem a little bit strange to not run max Speed, but hear me out. at 218 Speed EVs and a Jolly nature, Beedrill outspeeds almost all of the unboosted metagame (leaving only Mega-Sceptile and a Jolly Mega-Aerodactyl), and a not insignificant amount of the boosted metagame (most importantly, Scarfed Timid Heatran). Outspeeding Jolly M-Aerodactyl, M-Salamance with a DDance up, or M-Sceptile isn't possible even with max speed investment and is not necessary in the case of Sceptile who cannot 1HKO us but who we easily 1HKO in return with Poison Jab, so why invest in more Speed?
So, why invest that back into Defense? Priority. No amount of Defense investment is going to save Waspinator here from Talonflame or M-Pinsir (or even, sadly, Bisharp's Sucker Punch), but I ran some calcs and found that that 38 defense is sometimes the difference between being 1HKO'd by things like, Mamoswine's Ice Shard, Azumarill's Aqua Jet, Lucario's E-Speed, and Diggersby's Quick Attack after Stealth Rocks, and staying alive to Turn and Burn out. For example:
244 Atk Life Orb Mamoswine Ice Shard vs. 0 HP / 0 Def Beedrill: 173-204 (63.8 - 75.2%) -- 6.3% chance to OHKO after Stealth Rock
versus...
244 Atk Life Orb Mamoswine Ice Shard vs. 0 HP / 38 Def Beedrill: 160-188 (59 - 69.3%) -- guaranteed 2HKO after Stealth Rock
M-Drill is ludicrously fragile, so every time he gets to smack something with U-turn instead of switching out it's a powerful boost for your team considering that against many of those threats (Diggersby, Mamoswine, Weavile) he's dealing 75%+ to them with U-turn which is a huge momentum gain.
All that being said, I'm not sure he's got what it takes to make it in OU. His list of things he hates, in addition to those aforementioned Priority users (plus Offensive M-Scizor and anything that's picked up a boost from DDance/Swords) also includes Landorus-T, Stealth Rock, Sableye, M-Aerodactyl, Prankster T-Wave, Stealth Rock, Burns, Stealth Rock, numerous Pokemon immune to or dual-resisting his STABs, and finally Stealth Rock. It's a lot of support to find on your team for a momentum generating Pokemon yet, and I still haven't found the perfect set-up although Rotom-W seems like a good starting place.