I tried to find out myself, and went to the damage calc on pokemon showdown, slapped in Mega Mawile (Usage) and added Mega Ampharos, reset its EVs and added speed EVs until my speed stat was higher than Mawiles, which came out at 44 speed EVs to make Ampharos' speed 71 compared to Mawiles 70. Is that accurate?
While I appreciate Mawile would be a threat to Ampharos (Play Rough being an EASY OHKO) Wouldn't I just swap out? Even taking a sucker punch ahead of volt switch would leave me taking 50-60% damage according to the calc =/ While at best all I can manage is a 2HKO, so wouldn't I be better off swapping out as soon as mawile came in? (though surely mawile would be a higher teir anyway?)
Also, is there a fast way to check what tier pokemon are in? I find the way smogon write their lists a bit tedious, I'd quite like a drop down menu where I select a pokemon and it tells me what tier it is in xD