Warning: autistic amounts of "muh patterns".
I made a spreadsheet comparing the bonuses the speedy Megas got to their base Speed stat.
I defined "speedy" as at least base 100 Speed after mega-evolving, and excluded outliers with abilities that skew their stats (Speed Boost, Pure Power), legendaries, and Beedrill for being too minmaxed.
Then, I made a polynomial fit to the boost in speed as a percentage of their pre-mega Speed (e.g. a pokemon going from 80 Speed to 100 Speed gains 25%), and got a formula for reasonable Speed gains for a speedy Mega evolutions. It's off by at most 8 base Speed, in Kangaskhan's case, but it's a good framework, I think.
As an example,
>>26780624 this Haxorus and Hydreigon is ridiculous. The mons around 95-100 base Speed gain 20 base Speed, and at Haxorus's 97 the formula predicts 116 and 117, with the inaccuracy factured in they realistically would hit 125 Speed.
Of course, you can't ACTUALLY predict Gamefreak, and actual shitmons do deserve minmaxing.
