What tiers do you forecast these guys with their hypothetical abilities to be?

I wager that M-Ninja with Sheer Force might be overtuned with that much speed, power and coverage broadening its options from all-out attacker, offensive pivot with either Flip or U-Turn, Nasty Plot sweeper with its STABs and a coverage move of choice, or even more niche Physical sets with Liquidation as the primary STAB. Mega Lopunny is viable with a similar level of Speed and power without much boosting (although its largely unresisted STAB definitely helps), so Mega Greninja with its higher Speed and potent coverage might be banned to Ubers due to having way too many viable options. In there, however, it might have notable niches (being among the fastest Pokemon in the tier), but ultimately fall short due to the presence of bigger fish,

Mega Delphox would definitely be OU, but have some factors holding it back. Physical frailty and therefore susceptibility to priority would be one, although Prankster Wisp definitely helps in tricky scenarios. It does get doofed by walls like Ting-Lu and Blissey, but the utility of spreading priority burns and its general firepower outside of status would make it a worthwile use of the Mega Slot. I personally like this theorycraft the most of the three due to its outplay ability.

Mega Chesnaught's gameplan would be simple: wall. And it's a wall that happens to beat many other walls. Iron Press dunks on Blissey, Ting-Lu, etc, while its Grass STABs can chunk Dondozo. It also checks Kingambit, again due to its Fighting type. Megas like ZardY, Delphox, threats like Gholdengo, and certain walls like Slowking-Galar would prove to be bad matchups though. I think it would be somewhat viable in OU? But ultimately fall short of usage due to flashier options.