DCB,
A friend of mine, who is retired and has the time (lucky devil) has fire-lapped a couple of rifles with what I call dramatic results.
One was a Rem 700 in 30-'06 and the other was a Ruger in same caliber. Both guns exhibited rough spots (by feel) when slugging the barrel, prior to the fire-lapping (the Remington seemed worse). If you've ever pushed a lead bullet through a barrel, you know what I'm talking about.
A long story made short, The Remington group went from approx 1 1/4 in @ 100m down to slightly under 3/4 in @100m. The Ruger didn't show that much improvement, but I seem to recall it came down from about 1 in to slightly under 3/4 in.
All loads fired were handloads meticulously replicated before and after fire-lapping according to NECO kit directions.