You asked about civil cases, specifically, but there's a difference between civil and criminal that's important here. In a criminal case, there's the "finding" phase of the trial (conclusion: guilty or not-guilty) and the "sentencing" phase. Modifications will be relevant to sentencing. If you are found guilty of murder because you shot someone, then the fact that you modified the gun to be extra-good at killing people would be something that the prosecutors would use against you. It is not relevant at all to the "finding" phase, in my opinion.
In a civil case, there is no such dichotomy. The trial is all one thing. Again, if you did something wrong that you ought to pay for, then the fact that you modified the gun to be especially efficient at killing someone could be relevant. It could be argued that it was evidence that you were ready, willing, and able, and therefore intended, to kill. That would be relevant to the issue of damages, and especially punitive damages.
If you don't have a legally sufficient and good reason to kill someone, don't think about the gun, don't touch it, don't gesture towards it, don't refer to it in any way. Forget it's there unless and until it is really necessary to kill someone. And should worse come to worse, and that necessity arises, then don't try to use the gun as a negotiating ploy, as a tool of intimidation, or as a warning. While you can always change your mind and put the gun away, at the time you pull a gun out of its holster, you should already have a legally sufficient and good reason to kill someone.
In other words, at that point, you should be ready, willing, and able.
As Southernboy pointed out, the big issue in the case is going to be the fact that you shot someone. So if you don't have to dance around that issue or pretend it didn't happen, then you can confront the evidence squarely, readily admit the shooting, and be able to explain (with my assistance) why it was the right thing to do. In that case, the fact that you've modified the gun to be more efficient is completely irrelevant. It doesn't matter how you killed Badguy, what matters is why.