The question raises several issues.
First, keep in mind that many modifications make a firearm safer or address minor manufacturing flaws (e.g., a trigger that is not smooth) or merely accelerate the firearm to a state that normal use would have created over time.
Second, in the context of Tennessee's criminal law, the issue is whether deadly force was justifiable, if it was the evidence of the modifications should not be an issue.
If, however, it is a case of criminal recklessness or negligence (not self defense), then perhaps the fact that the firearm was modified might be an issue if the modifications allowed the firearm to be discharged when an unmodified firearm would not. Thus, weapons modified for competitive use with very light triggers would be a possible concern on the street.
Third, we must also consider how this issue might play out in a civil trial of negligence, recklessness or even excessive force. Here the issue is whether a "reasonable person" would have acted under similar circumstances (e.g., the carrying of a firearm - perhaps a known dangerous instrumentality) if the person knew or should have know of the characteristics of the modifications. The basic inquiry is whether the firearm was safe (e.g., compare a lawsuit about a car with bald tires or one involving towing too large of a trailer) and whether is was being transported in a safe manner.
Me, I tend to stick with the factory mechanics, a little fine tuning by a professional gun smith is ok, commercial grips and maybe even new sights.
Tennessee Firearms Association, Inc.