Actually, if you're running Modest, which is what most people will most likely be running because of the already huge ass Speed, it still out damages standard Life Orb Alakazam.
252 SpA Life Orb Alakazam Psyshock vs. 4 HP / 252 Def Blissey: 347-409 (53.22 - 62.73%) -- guaranteed 2HKO after Stealth Rock
252+ SpA Mega Alakazam Psyshock vs. 4 HP / 252 Def Blissey: 357-420 (54.75 - 64.41%) -- guaranteed 2HKO after Stealth Rock
So with Mega Alakazam you're getting the same damage as Timid Life Orb while also having blazing Speed and Trace, which is a neat ability that will have a lot of cool uses.
I'd also rather assume Jirachi is not getting a type change then assume it is, so I'm just going to leave it how it is.
Sorry for the late reaction on this post but I would like to say that the comparison isn't valid. I think you should compare a modest nature in both cases instead of timid vs. modest. Because a Modest Life Orb Alakazam does outdamage MegaZam. I am pretty sure that the point has already been brought up by someone here but I just felt like saying. I do understand your comparisation since Alakazam doesn't naturally outspeed anything and really benefits from the timid nature, while MegaZam has 150 speed which outspeeds enough.
252+ SpA Life Orb Alakazam Psyshock vs. 252 HP / 252 Def Blissey: 380-448 (53.2 - 62.7%)
252+ SpA MegaAlakazam Psyshock vs. 252 HP / 252 Def Blissey: 357-420 (50 - 58.8%)