Roost gets priority from Prankster.0 Damage. Murkrow will never outspeed an M-Manectric ever. Even at Max+ Speed carrying a Choice Scarf. The damage that will always happen is as follows:It's a damage calc but I don't know how to factor in type change so goes here I guess.
How much does a Thunderbolt from M-Manectric do to an Eviolite 252HP Murkrow after Roost?
252 SpA Mega Manectric Thunderbolt vs. 248 HP / 8 SpD Eviolite Murkrow: 390-462 (120.7 - 143%) -- guaranteed OHKO
M-Manectric will OHKO a Murkrow no matter what.