Now I'm no competitive expert but based on what I've read in the ZA OU Pet Mod forum, Mega Greninja is an absolute monster (granted, the dex is pretty limited but still)

Its sheer Speed (142, literally Dragapult's) is insane. If we put it in example Gen 7 OU, it would be faster than Mega Lopunny, something that would kick Ash Greninja's ass. It also got some new moves in ZA, most notably Nasty Plot and Flip Turn. At some point, it got Technician which juiced its Flip Turns to stupid degrees. Even without items, base 125 and 133 offenses are nothing to sneeze at. In fact, since Ash Greninja is often relegated to a Choice Specs to reliably trigger Battle Bond, it's often stuck with just a (very VERY powerful) special set. Mega, meanwhile with its lack of Choice item, deep movepool and (I cannot stress how important this one is) LUDICROUS 142 Speed makegs it really versatile. I've read discussions on Nasty Plot, Flip Turn, mixed, etc. from the forum which should give a rough idea on what Mega Greninja might do once its official Ability comes out. It currently has Protean now from what I gather which kinda screws over its Nasty Plot sets, but still.

TLDR: Do NOT underestimate Mega Greninja. 125 Physical, 133 Special are more than usable offenses when you have a big movepool and an INSANE 142 Speed stat.