I think since this set does rely on its surprise factor, it definitely needs as close to identical attack as a jolly Garchomp. You have like a 3/16 chance of exposing your set by eqing a Hippowdon for an impossibly low damage amount atm..
But honestly I think if you're running 300 speed, I still prefer Physical.. Here are some similar calculations for Adamant Life Orb Chomp.
vs Hippowdon: 357 - 420. 99+ on rng (1/8) is an OHKO with LO Outrage after an SD, but Hippowdon cant OHKO Garchomp. With one Layer of spikes you have a 50% chance to OHKO, plus the 80% chance Hippowdon will miss you.
vs Weezing: minimum 357 damage. Guaranteed OHKO.
vs Slowbro: 379 - 446 damage. SR or Spikes guarantees OHKO, 75% chance of OHKO otherwise. Also if Slowbro has enough Special Attack to KO Garchomp, it is guaranteed to be OHKOed by the outrage. Slowbro can not survive the turn in SS.
vs Skarm: LO Fireblast is a guaranteed 2HKO, SD fire fang is a guaranteed KO. ChainChomp obviously is better here..
vs Starmie: (304 hp no def??) Outrage does 322 minimum to no Def Skarm. Earthquake has a 5/16 chance to KO on a switch in SS, with one layer of spikes Quake is a guaranteed KO.
vs Gliscor: has a 1/8 chance to survive with absolute maximum defense.
vs Donphan: is a better counter.. 3hkoed by EQ.. Chainchomp is definitely better in this situation.
vs Blissey: Haha..
vs Bronzong: Bronzong who knows what crazy EVs they could be running.. But Fireblast will pretty much always 3hko. SD Fire Fang is a guaranteed 2hko.
vs Cresselia you do 357-420. With Stealth Rock and SS you have a 50% chance to KO, Cresselia will only have a 1/16 chance of surviving the turn.
It is a pretty good set, definitely better for Bronzong, Skarm and Donphan.. But Physical Garchomp is just so broken already.. I would worry that you will find yourself countered by things you really shouldnt be, and the speed hit is a big hit..
Have a nice day.