How much of a difference does this make?

I just do it, but I have never tested it against one I didn't.

I know it provides a more even interface for the barrel to rest on, but has anyone ever done a comparison?

I've never seen anyone post before/after comparisons, if that's what you're talking about.

It isn't something that should be needed with a quality upper, but if you're concerned about it and want to spend the money on the tool and the time using it correctly, it also shouldn't hurt.  Tip: the lapping tools should be used by hand, whether or not they look like you should be able to run them with your power drill.

I have never done it on any of mine, my guns shoot good and I have never found a need to.

I had the rear sight on BUIS adjusted significantly to one side on a pistol before I lapped the receiver. Afterwards, the rear sight was much more centered. Now they all get lapped.

If there is anyone I can think of that may have done before and after, or side by side testing, it would be Ritch Johnson. You might want to reach out to him and see what he has to say on the matter. Here's his website: http://ritchsprecisionguns.com/

I looked into it during my 6.8 build.

I first noted all it would do is center up the adjustments needed to zero. That's ALL it can do. It does not make the barrel more accurate - only the rifling, chamber, and load do that. All it does is make the nose more flat to match the barrel extension ring.

That means the ring was machined correctly in the first place, which is likely due to it being done on a lathe. And then, when it's screwed onto the end of the barrel that the threads match concentrically, which they do - mostly. We are still talking +/- .015". So we mate a mostly coaxial barrel with mostly perpendicular ring to the front of a mostly flat nose and clamp it down with a barrel nut. Hope they got the upper mostly coaxial. As I said, it's all =/- .015".

Second I noted most of the 6.8 builders who offered to square the nose did so EXTRA COST. They were not assembling their MOA guaranteed rifles by doing the work themselves up front. They just stuck in the barrel and tightened it down. And they still warranty the MOA.

All it does is center up the adjustments to the mostly coaxial bore line - =/- .015".

Want to ponder something? Nobody shoots the barrel to group it before they tap the gas hole and grind in the feed ramps. It would be nice to have one that was oriented to 12:00 when assembled. That service hasn't been revealed to me when I search for it. You get a barrel with whatever group it lands on - 3, 7 11, whatever. and then you still have to crank in adjustments to correct. Wouldn't it be easier to simply dial in whatever elevation correction it needed. Not happening. Yet.

So for the expense of buying and using the tool, I got a 6.8 that I have no idea is better centered up or not, and I get to square the nose on AR's when it seems the torque wrench is going over 80 foot pounds but I haven't got the teeth aligned for the gas tube. In that regard the tool did pay it's way.

I bought the tool because I got it for "free" using a military discount on the order. Oh Well. YMMV.

I have always had the tool, so I always did it.

I have never not done it.

Wow!  Actual before/after experience!

No sarcasm intended - your experience is very useful to me.  I can see going to the trouble to test each upper with a lapping tool, as in putting the tool in, turning it a few times, and seeing if you see any difference.  Maybe chalk up the front and see if the tool removes the chalk smoothly or not.

It also makes me wonder how many "your FSB isn't straight" rifles actually had out-of-square uppers...

I agree that a lapped upper can't "make the gun more accurate".  However, I do see an issue with a new build that requires a substantial shift of the sights in one direction or other to get it zeroed, and more importantly with that shift sometimes being vertical.  And statistically, that ±0.015" will be overwhelmingly very close to zero, at least in barrels from quality barrel makers.  But I agree that there's nothing "magical" about truing the upper's face except that it eliminates at least one source of frustration - sight alignment for zero on a new build.

Krieger's barrels are made with the gas port in a groove, not in a land or between the two.  That seems to be a positive thing for accuracy, at least the way I read Molon's comments in this thread.