I don't have the skills to be highly critical of stock fit, most will work for me. Perhaps as my skills improve stock fit will become more important and maybe it won't. Some highly skilled people need to have their equipment meticulously set-up exactly how they need it to be for them to perform at their best and without htat set-up they're an "also ran" at best. Others can take any quality equipment and perform just as well regardless of how that equipment is set-up. There's a famous 70's era Stock Car driver who was well known for being just as fast in a well set-up car as he was in a car that was poorly set-up. Skill can overcome equipment, when there's enough of it.
If stock fit is important, and I would suggest that it is to more of us than not, then hands-on is going to be the only way to chose.
RE: a guy's interest buying a rifle until finding out the owner built it: I find it interesting that how well it shoots is second in the minds of some guys than who built it. If it doesn't have a big name behind it, how it shoots is of no interest.
I get that there are a lot of hacks out there, both with and without a good reputation, and it is wise to be careful with an unknown builder. I have a desert chase/pre-run vehicle that supposedly has some parts on it from a well known fabricator that I'm having to replace because they are flat wrong. I bought the vehicle giving no credence to that fabricator claim, so I'm disappointed, but not hurt. I would buy a custom rifle with the same mental approach. Even with builder documentation until the rifle is thoroughly examined and proven to be correct and shoot well I consider it to be no better than if some hack in his garage assembled it.