no, honestly, guys... no need to replace serpent sting with viper sting on the main target. there are enough fights with adds in them, with adds that are casters - switch target with a macro: /target [add name] /cast viper sting /targetlasttarget
done. it will just cost you one GCD. why use freaking feign death, which is one full gcd, then use a mana potion, which is another gcd and then forfeit it to really dps important potions, like agility. and then just calculate how many times you will use these two gcd's and think how this will affect your fight.
yeah, you will have to learn the names of the adds and actually change the details of the macro prior to any fight, but that is not too much to ask for preparation. it is just how you manage yourself...
if it is a meter topping fight, just make friends with a druid and ask for an innervate as you will anyway just compete amongst yourselves.