[1] I am a lawyer. Of course, I'm not your lawyer, so I'm not giving legal advice. I'm providing comment on a general legal topic based on my training and experience.
[2] We really have to get over this "a good shoot is a good shoot" business. If you are on trial after a self defense shooting, someone doesn't think it was a "good shoot." Either the DA decided that he had something to prosecute or the grand jury concluded that there was probable cause to believe that a crime had been committed and that you did it. There is a dispute about whether the shooting was justified, and determining whether it was a "good shoot" is now going to be up to the judge and/or jury.
You don't have the final say as to whether or not it's a "good shoot." Sometimes, probably often, the evidence may be substantial that your use of lethal force was justified self defense; and if that's the case, things like modifications to you gun pretty much won't matter.
But there's no guarantee that if a "bad day" happens to you, when the dust settles everyone will be agreed that it was a "good shoot." Physical evidence may be equivocal. Witnesses may tell conflicting stories.
[3] If you're going to need to tell your story to a jury, in my view, certain types of modification, basically things like disabling a safety device, could make things tougher for you. These issues have been discussed at great length on this board. See the following threads:
http://www.thefiringline.com/forums/showthread.php?t=372759
http://www.thefiringline.com/forums/showthread.php?t=366434
http://www.thefiringline.com/forums/showthread.php?t=388901
http://www.thefiringline.com/forums/showthread.php?p=3586536
[4] I will not use a gun on which a safety device has been disabled, a gun which has been modified to have a very light trigger (something less than around 4 pounds, ETA -- I favor a 1911) or handloads for self defense or carry.
[5] It's really primarily a matter of how certain such things could affect the impression that members of your jury have of you. Things that might bother a jury can influence how the evaluate you, your testimony and the evidence. Remember that a big part of your legal defense will be you telling your story, and so the jury's take on you can matter. We know, from post verdict interviews of some of the jurors in the Harold Fish case (in gun friendly Arizona), that some of them were bothered by his use of a powerful gun (10mm) and JHP ammunition. (Fish was convicted of manslaughter and went to prison. He did win his appeal, and is now free. But the point is that the gun and ammunition affected how his jurors evaluated the evidence.)
[6] But other gun modifications, like full length guide rods, better sights, and the like, are probably benign and shouldn't be a problem. But it wouldn't hurt to have a good and non-technical explanation for why you did something.
[7] And about jury selection: Your lawyer will be busy keeping anti-gun folks, members of the Brady Bunch and the like, off your jury. But the prosecutor will be systematically keeping people who are pro-gun (like NRA members, active recreational shooters, maybe anyone who has a gun) off the jury. So your jury will pretty much be made up of folks who have no special knowledge about, nor interest in, guns. Technical explanations will go in one ear and out the other.
[8] And a good general rule in court is that the less you need to explain, the better off you'll be.