Ubiq,
The problem with that svara nowadays is that milk is no longer bottled (or cartonned) at the cow. Rather all the milk is gathered together in vats and then cartonned. Therefore if Treifos are known to be greater than 1.66% (the stomach piercing procedure alone effects approx. 14% of milk cows and is recorded cow by cow on farms), their milk will not be battul and and the milk will be treif (m’drabbanan).
The only way to get out of this problem is to hold like Rabbi Belsky that the stomach piercing procedure does not render the cow a treifa altogether.