Any relevant calcs that the Def investment allows it to avoid? The extra SpDef that HP investment provides is useful to allow it to set up. I did put Psyshock as the first option but Thunderbolt does hit Ho-oh a bit harder. I stand by what I said about it only being able to set up once. You are playing with 5 Pokémon until you manage to remove all their Xerneas counters, some of which are very bulky and difficult to take out, unlike Arceus-Ground or CM Ogre for example, who can set up repeatedly over the course of a match.
Considering 104 speed necessary, so 152 EVs to put :
Choice Band Scizor 252+ Bullet Punch vs 152HP/0def Xerneas : 341-401 (79,1%-93,0%) - 39,5% chance to OHKO after SR
Choice Band Scizor 252+ Bullet Punch vs 0HP/152def Xerneas : 292-344 (74,3%-87,5%) - never OHKOed after SR
Life Orb Aegislash 252+ Iron Head vs 152HP/0def Xerneas : 436-513 - always OHKO
Life Orb Aegislash 252+ Iron Head vs 0HP/152def Xerneas : 374-440 - 29% chance to survive without SR
Leftovers Aegislash 252+ Iron Head vs 152HP/0def Xerneas : 337-396 (78,2%-91,9%) - 32,1% chance to OHKO after SR
Leftovers Aegislash 252+ Iron Head vs 0HP/152def Xerneas : 289-340 (73,9%-86,9%) - never OHKOed after SR
Well, that's not that impressive, but that's some important facts. I guess the two spreads are viable.