The thing with megas is that they had diminishing returns in XY: Mega Tyranitar was a 700 BST behemoth that was... another set for Tyranitar. Same for the Mega Mewtwos; highest stat total AND individual attacking stats in the franchise... and they were each a set for Mewtwo. Mega Garchomp had that intention as well, but the speed drop was too much for it to handle. The Mega Latis were also made for XY, and said Megas consistently work as a set for the base form.
The Megas for Scizor, Pinsir and Heracross pretty much placed all 3 of them as roughly equally viable and divergent performance, as it is usually the intention with usual choices for version exclusives. And most of the stuff in the 480-540 BST range has perfectly fine megas. Mega Gengar has a busted ability for its power tho; Cursed Body should had been implemented there to begin with.
Mega Mawile was overkill tho. After mega evolving it has Raichu tier stats, so, Huge Power to compensate and give it the highest actual attacking stat with the best typing in the game. Mega Sableye as well, cue a throughly minmaxed Raichu tier 'mon. Both of those designs would have worked as 510 BST 'mons with the abilities of the base forms. Mega Audino is a bit of a waste, but, the 450 BST area is a bit of a limbo; for each Crobat or Annihilape, you get a Dusknoir or a Wyrdeer. You get Mega Absol and Mega Sharpedo, but also Mega Banette and Mega Camerupt.
And, of course, the high end ORAS Megas weren't balanced compared to the XY ones. Mega Salamence and Mega Metagross, unlike the other mega pseudos, became straight upgrades, mostly because they, rather than investing the stat increase into ludicrous numbers, just patched speed and defenses, while letting their new abilities do most of the extra damage.
And, of course, the Primals (which are kinda supposed to be OP, but, they aren't really balanced with each other), and Mega Rayquaza, which could be balanced around the primals if it wasn't for it not needing to hold an item.
The concept of a 100 BST boost with some type and ability changes is neat, but, the key point IMO are:
1. Have those changes matter more than an actual evolution would do; plenty of 'mons where an 80 BST boost up to the 530 range works far better than getting to 550 while disallowing you from using a different mega. Stuff like Beedrill should be an exception (and it works well to make those weak 'mons do something in lower tiers).
2. In high BST 'mons, have the stat increase be one that, rather than upgrading the 'mon, works as a set for it. Mega Tyranitar was great for DDtar, but Tyranitar had other sets. Mega Gyarados changed type dramatically, as well as ability; a nice discard and draw.
Those are, all in all, formes, and new toys for old 'mons.
tl;dr? Bisharp should have gotten a Mega, Mawile should have gotten a regular evo.