No Merc is hitting for 5k-6k every round. Tracer missle doesn't hit that hard, Heat seeking missles can hit that hard occasionally though. Not sure what you're basing anything you're saying off of though. I've been on plenty of raids where our DPS sorc (0/13/28) get second aggro over our DPS Merc (5/31/5), and it seems to fluctuate between the 2 based on how often they crit. I will lean and say our Merc does pull it about 60% of the time. I've never seen our Sniper (22/16/3) pull second aggro.
Yes, the mercs do crit for 5-6k every shot. I believe you are right though that was heat seekers, and i checked and double checked last night when we ran EV again. Yup, same exact order. Noone pulls aggro off the tanks, but the one dps who is 2nd on the aggro list always gets blasted by that first boss every 30 seconds or so.