I could get behind the idea of Mega Zam moving up to A, albeit as one of the lesser A ranks. The most immediate, albeit probably not the most fitting comparisons I can make are to Mega Manectric (as a fast, coverage backed Special Attacker) and Zard-Y (Powerful Specially oriented Wallbreaker). Manectric boasts Intimidate and access to Volt Switch, making it hard to defeat since it pivots out of unfavorable match ups while being able to blunt attackers. Zard-Y has fairly good coverage for a wallbreaker, and absurd power behind its Fire Blast under Sun, with even some common resist heavily dented on the switch
252 SpA Mega Charizard Y Fire Blast vs. 72 HP / 0 SpD Latias in Sun: 126-148 (39.4 - 46.3%) -- 30.5% chance to 2HKO after Stealth Rock
That said, Alakazam has some noticeable advantages over the two.
- A useful speed tier even Pre-Mega. Base 120 is faster than a good deal of the Metagame even before Zam hits the 150 for outpacing slower Scarfs. This means that against offense, Alakazam is less pressured to find a "safe" Mega turn, where many other Megas have a risky turn with their pre-Mega speed, which can cost them match-ups they'd win if already evolved (such as Diancie). Zam has mons like this too, such as Mega Lopunny, but these mons need to already be in, since they can't stomach a switch hit and won't often outspeed afterwards.
Alakazam's speed is what really makes him. Trace wouldn't always be a particularly great ability vs Offense because most users of the ability are slow enough to be Revenge Killed by a faster mon before netting more than one kill. Zam outspeeds every user of the abilities he'd like to steal, from Sheer Force to Adaptability. This has an indirect advantage as well: the opponent has to moderate their use of Ability dependent attackers, because if Alakazam comes in for the revenge, stomaching a Base 175 Sp Attack backed by that ability could be game right there barring priority, which Zam can still mitigate if it predicts a switch or disrupting attempt and runs Substitute. Alakazam's power is also fairly high, and he can put Trace to use against balance and Stall as well, snatching abilities like Diancie's Magic Bounce or Heatran's Flash Fire, not to mention having set variations such as CM or even Lure sets (I've seen Knock Off carried to get the 2HKO on Eviolite-less Chansey).
Maybe I'm overselling it admittedly, but Alakazam is an interesting mon in that he can be played relatively the same and put in a similar amount of work against about every playstyle: Tracing to Revenge Weather Sweepers, dismantling Balance and Stall Cores with high power and coverage, outspeeding and smacking many offensive threats, all while Trace lets him play with the opponent's own toys. The only other Mons that stick out as doing that to me are Metagross and Gengar: Most others either need to run a different set (Talonflame, Keldeo, Altaria) to cover a different playstyle, or simply do not fare as well against one over the other (Landorus-T v Stall). Plenty of mons match up better against respective playstyles, but Alakazam is proficient enough with one set that he doesn't make you weaker against one playstyle to the point of trying to amend it actively in another teamslot. Alakazam isn't the MOST splashable, but at the same time it's a mon that doesn't have to be built around or tailored too specifically to do its job.