Drampa sucks. The reason Drampa sucks is because it has such a low speed tier and bad Defenses. Bezerk is only good if you can out-speed your opponents or take a few hits, which isn't something Drampa can do. Drampa's Defenses aren't good enough to take enough hits, even with max investment, so, combined with bad Speed, Drampa goes down quick, and substitute doesn't help that. Sure, you can heal with Roost, but it isn't very hard to knock down Drampa, especially in OU. If a Pokémon is in a lower tier than you are playing, it is there for a reason. Don't use Drampa in OU. :P
Here are a few calcs:
252 SpA Life Orb Teravolt Kyurem-Black Ice Beam vs. 252 HP / 252+ SpD Drampa: 276-328 (76.6 - 91.1%) -- guaranteed 2HKO
252 Atk Choice Band Kartana Sacred Sword vs. 252 HP / 252+ Def Drampa: 304-358 (84.4 - 99.4%) -- guaranteed 2HKO
Most OU threats will 2HKO Drampa and prevent it from subbing, or will figure out you are Physically Defensive/Specially Defensive and send out another Pokémon to compensate. If you split Defenses and HP, more threats will OHKO Drampa. :P
Yes, Substitute activates Bezerk if Drampa ends up with less than half HP. :P
TL:DR: Drampa+Substitute activiates Bezerk. Don't use Drampa in OU, it will not go well. You strategy may work in the PU Tier, the Tier Shift Format, or maybe the Mix and Mega Format, if you really want to use it somewhere. :P
Hope I Helped!