I'm about to put a couple rifles together and I've noticed that Compass Lake offers a "matched bolt" service for best accuracy.

How much accuracy is gained?

What exactly is involved? Is it a DIY thing, or is it black magic only performed by those who have been building 1/4 MOA rifles for decades?

Gotta know, ya know?

I'm not familiar with Compass Lake nor their "matched bolt", but I'd venture to guess that it is nothing more than a bolt that falls within certain headspace parameters.

Hmmmm. I had visions of them hand lapping and aligning the bolt lugs to the barrel lugs.

So, it that is what it is, what sort of accuracy difference would it make vs. jputting it together myself and just using my go/no go headspace guages to make sure it won't blow up?

I think CLE sets minimum headspacing w/ their match bolt option.  They may also tighten up endspacing too, best thing to do is ask the man that does the chambering work at CLE.  

You can't change headspacing on an AR with go no-go gagues, well without a lathe and a chamber reamer.

On how much more accuracy you can expect it's not really cut and dry.   A minimum headspaced chamber can hold the case more concentric in the chamber and give an advantage.  This becomes less of an issue when you handload because you can set back the shoulder based on your chamber when you resize.  Tightening up endspacing can make your brass last longer when you handload by minimizing the amount of sizing action the rifle does to the brass when the bolt closes and forces the round into the chamber.

+1 for it probably just being a properly headspaced bolt