You're calcing with a special attack increasing nature, which has already been stated to be bad as you end up being very slow and dying against things you should check.
You can still get the same result with a +speed nature and more EVs but it's a conceptual argument really. Not to mention most 100 tier pokemon commonly don't run speed that Glalie should be up against like Mew, Celebi and Tentacruel, and the only Jirachi that do run speed tend to be scarfed.The things that do run max speed Glalie has no business trying to speedtie anyway like Charizard or Medicham. Mega Glalie should be able to function perfectly well enough without max speed.
Currently the fastest relevant threat would be Timid Hydreigon which is rising in combination as a core with Mega Metagross. You'd need a speed of 325 to beat, unless scarfed.
Next would be Kyurem-B and Sharpedo before megavolving, but it tend to Protect to get a speedboost so that's kinda moot. 318 there.
312 for Mega Garchomp and Landorus-T
303 for Excadrill
288 for Gyarados, Altaria, Mamoswine, Dragonite, Venusaur, Goodra, etc.
A +speed nature and 160 EVs can hit 303 for Excadrill if you want, but is Jolly without a scarf or Sand Rush even common?
Otherwise the minimum speed Glalie should have is for Jolly Mamoswine and Dragonite.
This takes 104 EVs and a positive nature, or 208 EVs with a neutral nature. Since Glalie's offensive stats are higher, I propose a neutral speed nature. So this is the spread I'm hypothesizing for mixed attacker.
Glalie@Glalidite
Refrigerate
Rash 220 At / 80 SpA / 208 Spd
~ Freeze Dry
~ Explosion
~ Earthquake/Ice Beam
~ Double-Edge
220 Atk Refrigerate Glalie Explosion vs. 252 HP / 88+ Def Ferrothorn: 328-387 (93.1 - 109.9%) -- 56.3% chance to OHKO
80+ SpA Glalie Freeze-Dry vs. 228 HP / 0 SpD Azumarill: 252-296 (63.3 - 74.3%) -- guaranteed 2HKO
220 Atk Refrigerate Glalie Explosion vs. 0 HP / 0 Def Thick Fat Mamoswine: 294-346 (81.8 - 96.3%) -- guaranteed 2HKO
80+ SpA Glalie Freeze-Dry vs. 240 HP / 0 SpD Tentacruel: 180-212 (49.8 - 58.7%) -- 69.5% chance to 2HKO after Black Sludge recovery
220 Atk Glalie Earthquake vs. 240 HP / 244+ Def Tentacruel: 192-226 (53.1 - 62.6%) -- guaranteed 2HKO after Black Sludge recovery
220 Atk Glalie Earthquake vs. 248 HP / 0 Def Heatran: 384-456 (99.7 - 118.4%) -- 93.8% chance to OHKO
220 Atk Refrigerate Glalie Explosion vs. 4 HP / 252+ Def Eviolite Chansey: 648-763 (100.9 - 118.8%) -- guaranteed OHKO
80+ SpA Glalie Freeze-Dry vs. 252 HP / 4 SpD Skarmory: 138-163 (41.3 - 48.8%) -- guaranteed 3HKO after Leftovers recovery
80+ SpA Glalie Ice Beam vs. 252 HP / 4 SpD Skarmory: 178-210 (53.2 - 62.8%) -- guaranteed 2HKO after Stealth Rock and Leftovers recovery
220 Atk Refrigerate Glalie Explosion vs. 252 HP / 252+ Def Skarmory: 279-328 (83.5 - 98.2%) -- 68.8% chance to OHKO after Stealth Rock
220 Atk Glalie Earthquake vs. 252 HP / 4 Def Jirachi: 202-238 (50 - 58.9%) -- guaranteed 2HKO after Stealth Rock and Leftovers recovery
220 Atk Glalie Earthquake vs. 4 HP / 0 Def Bisharp: 202-238 (74.2 - 87.5%) -- guaranteed 2HKO after Stealth Rock
220 Atk Refrigerate Glalie Explosion vs. 4 HP / 0 Def Bisharp: 244-288 (89.7 - 105.8%) -- 75% chance to OHKO after Stealth Rock
220 Atk Refrigerate Glalie Double-Edge vs. 252 HP / 252+ Def Mega Latias: 300-354 (82.4 - 97.2%) -- 62.5% chance to OHKO after Stealth Rock
This seems like good mixed potential. Explosion is still strong enough to nearly guarantee a OHKO against many defensive behemoths and personally dismantles a few FWG cores between Freeze Dry and Earthquake. The only reason to consider Ice Beam is that the extra BP can actually turn certain 3HKOs into 2HKOs versus certain switch ins like Skarmory or Mega Heracross for example, without having to resort to Explosion too early. But Earthquake is far too important for numerous Steel switch ins. Standard Jolly Metagross is 2HKO even after megavolving and outsped too for that turn. Jolly Touch Claws Bullet Punch also cannot OHKO Glalie from full so that's good insurance to know that if Metagross took at least 50% from your EQ then it's safe to EQ again.