well, I have not had mine on a dyno, either before or after so I have no solid figures to show....
however, I can see and feel some differences. I had a 3" DP and all the mods in my sig before the 4" DP, it was the last mod done, and I had about 8,000 miles on the rest of the mods, towing (20,000 lbs GVW combined) so I did have a real good idea of what the 3" did.
the biggest difference is that the egt's cool down much quicker when you take your foot outta it, and it cools better at all rpm's, about 100 degrees. and yes, it pulls better, especially down low in the rpm range, it comes "on the pipe" earlier, and I can now run more manifold pressure in overdrive pulling the smaller humps on the hiway at 60 mph than I could before as it just got too hot before. it's helped that area considerably. I used to limit the boost to 8 lbs manifold pressure, because it got to 1200 degrees there, now, I can run 12 lbs boost at the same rpm and it still only gets to 1200 degrees, so it's made a solid difference there.
the rig feels more responsive, and doesn't heat as much, or as quickly, so I'ma happy camper.
and for those of you who may contemplate doing this, we had a complete shop to do this in, and it was still a complete PITA. cutting the floorboards out was the easiest part..... the hard part was building a cover for the hole, and figuring out how to heat insulate it .....