it has allowed me to kill the pokemon you just mentioned who survived my double edge as well as finishing some bulky waters and cresilla (pretty much anything just bulky to hang on)
"81% chance to OHKO". Double Edge is better by far. Keeping 100% accuracy and stronger (2 turns hits harder than Giga Impact, duh), there's no reason in hell to run Giga Impact. I'll get my calcs up:
252 Atk Choice Band Staraptor Giga Impact vs. 252 HP / 252+ Def Cresselia: 220-261 (49.5 - 58.7%) -- 71.1% chance to 2HKO after Leftovers recovery
252 Atk Choice Band Reckless Staraptor Double-Edge vs. 252 HP / 252+ Def Cresselia: 211-250 (47.5 - 56.3%) -- 28.9% chance to 2HKO after Leftovers recovery
Double-Edge wins due to Cresselia (lets just say that it's just a dummy) Giga Impact has to use THREE turns to kill, as the 2HKO says after Leftovers, it doesn't include that Cresselia gets the recovery twice. Double-Edge can just go for it though, having the chance to 2HKO if the Cresselia is a dummy.
252 Atk Choice Band Staraptor Giga Impact vs. 252 HP / 252+ Def Hippowdon: 222-262 (52.8 - 62.3%) -- 99.6% chance to 2HKO after Leftovers recovery
252 Atk Choice Band Reckless Staraptor Double-Edge vs. 252 HP / 252+ Def Hippowdon: 213-252 (50.7 - 60%) -- 85.9% chance to 2HKO after Leftovers recovery
Do I really need to explain this?
Double-Edge once again wins, due to Giga Impact taking that recharge turn. Double-Edge can just go for the 2HKO, once again. Now against a real
attacking back opponent.
252 Atk Choice Band Staraptor Giga Impact vs. 252 HP / 252+ Def Clefable: 304-358 (77.1 - 90.8%) -- guaranteed 2HKO after Leftovers recovery
252 Atk Choice Band Reckless Staraptor Double-Edge vs. 252 HP / 252+ Def Clefable: 292-345 (74.1 - 87.5%) -- guaranteed 2HKO after Leftovers recovery
4 SpA Clefable Moonblast vs. 4 HP / 0 SpD Staraptor: 150-177 (48 - 56.7%) -- 87.5% chance to 2HKO
So, once again,
Double-Edge wins once more. Staraptor will go use Giga Impact, take a hit from Clefable's Moonblast, then (most likely) die to the second one due to the recharge. On the other hand, Double-Edge almost kills, Clefable hits back with a Moonblast, and Staraptor can freely attack again, winning, unlike Giga Impact.
I believe, that was a fine explanation, I don't see anyway Giga Impact is better. Even if you kill a Mon with it, a new Mon can come in, and has a free turn to kill Staraptor.