personnaly i dont think using a mixed grip makes a drastic difference... i use mixed myself for the fact that 1. it lets me work more weight 2. it strengthens my grip in the position that i use it in competition... (though im sure for many that is not a concern, just justifying my use) and my back/traps is pretty damn symetrical...
are you using dumbells? if so, switch to bar... the 'bells give each arm a seperate opportunity for their own ROM... the bar will give their paths a better chance at simultanious function....
if you are using bar... and not using a mixed grip... i dunnno...
is the larger trap on your weaker shoulder side? thats the only other thing im thinking atm...