The issue of whether or not Swift goes too far in "A Modest Proposal," is an issue of taste. It's a satire, of course, so the speaker has much freedom in what he writes. He also uses irony. Both satire and irony often use exaggeration as tools to accomplish their purposes. So, again, the writer has a great deal of freedom to work with.
The question is, though, even within the accepted norms of satire and irony, whether or not what Swift does is in bad taste. His proposal and his descriptions, etc., are certainly grotesque, and his central idea strikes at the core of humanity--cannibalism.
Ultimately, however, though the essay probably is in bad taste, Swift decides that using bad taste is worth the risk. The essay shocks the reader, and probably would not have done so if it weren't in bad taste. The bad taste of the proposal is part of the point.
In the end, each reader probably decides whether the essay is in bad taste, whether it goes too far. I love the wit and the humor and the satire and the irony. But that doesn't mean another person wouldn't find the essay in bad taste.
Swift's purpose is to shock his readers with his very "immodest" proposal. In Ireland at this time, there is extreme poverty, overpopulation, and an unfair balance of trade with Great Britain. He suggests that the Irish should sell their children as food to reduce the overpopulation and poverty. Of course, the reader is aghast at such a suggestion, and Swift's purpose is to glean this reaction from the reader. He wants us to know that people should not be dehumanized as a number but should be treated with kindness and compassion.
I don't think he goes too far because he effectively uses irony and satire to get his point across. Sometimes, this is what it takes to make people sit up and take notice of the existing problems in society.
The purpose of Swift's satirical essay is to call attention to the problems that were being experienced by the people of Ireland. He wanted the English (who ruled Ireland) to realize what they were doing and to put in place reforms that would solve the problems they had helped to cause. So that is the purpose of the essay and what he is trying to reform.
I do not really see how you can go to far in a satire. I mean, he's not really suggesting that people actually do this, so how can it be going to far? So no, I really don't think he has gone too far.