I don't know if this was asked before, but do Pokemon after mega evolving have different EV yields? I thought it was possible because:
The change in form take's place before anything (other than switching) and hence the change would in EV yields can always happen before the said Pokemon is knocked out.
The base stat changes recieved is a titanic 100 BST (base stat total) more, and even abilities change to better suit they're function, sometimes completely changing they're role altogether (like Heracross now becoming a bulky attacker instead of a sweeper), so there is a chance the EV yield would change too.
All in all this is just a question out of curiosity, but it may hold significance too.