I think the impact can best be summed up by the incidents that helped get the trace data removed from public consumption in the first place. VPC used to regularly publish a "Top 10 Crime Guns" list based on ATF trace data made available to the public. This was regularly reprinted by "news" organizations like Newsweek and Time even though
several pro-RKBA academics and the
ATF itself pointed out that just because a trace request was made for a particular firearm, didn't necessarily mean it was involved directly or indirectly in a crime.
If just the trace aspect of the Tiarht Amendment is removed, then you can expect to see more of that again. However, the Tiarht Amendment also codified the 24-hr destruction of legitimate purchases in NICS records and also loosened restrictions on some FFLs to make it easier for small FFLs. If they completely reversed it, that would hurt.