This is going to vary from state to state so it is best to check what a specific state's statutes or case law says on this one. Consult with an attorney in your state who has experience in defending people with firearms charges and who is sympathetic to Second Amendment issues. In my state this is not an issue at all.
This topic comes up often on some of the other websites, glocktalk.com in particular, and it generates response from don't even think about touching your Glock to turn it into a race gun and go for it (yes that's a little far fetched on my part). Fact is, as I mentioned it's going to vary from state to state. I was quite concerned about this about four or five years ago and consulted an attorney who has a track record in my state. His response was that the 800 pound gorilla in the room is not going to be whether or not you modified your trigger but rather if the shooting was justifiable or excusable. We have an affirmative defense in Virginia which means that you will likely be put on the stand by your attorney and you will admit that you deliberately shot Mr. Smith and under the same set of circumstances, would do the same thing again because you were in fear of imminent serious bodily harm. Hard to show that a little trigger work caused an unintentional discharge when you just admitted your actions were sober and deliberate, and you fully meant to shoot your assailant.
So if in doubt, meet with an attorney. And make sure he's knowledgeable about these matters and is pro-self defense.