.40 barrels
I can't say that I would endorse shooting .40 S&W in the 10mm barrel. As noted, the headspace issue with the shorter case is just not "right". Why stress a gun part? And why disregard the many warnings over the years about using ammunition only intended for the particular firearm?
I'd go with the .40 Lone Wolf barrel. Keep in mind that the LW will allow one to shoot lead bullets, which are cheaper than a jackedted slug. I'd speculate that one could get ahead of the initial cost of the barrel in less than 1000 rounds. Plus, brass is way more plentiful, to the point I'd state that .40 brass (and 9mm) is just about free if one spends just a little time picking the stuff up at local ranges. I have NEVER bought any .40 or 9mm brass and have gobs of the stuff.
You hear the 10mm bashers always refering to weak 10mm ammo and the .40 is near equal in a lot of loads. There is truth to that, some 10mm is soft, but there is another side of that arguement. Some .40 ammo is darn soft too. I've chronographed boxes of .40/180 FMJ that did not break 900 fps from my Browning HP.
Regards the ".40 in a 10mm" is the same as ".38 in a .357" comment. No it is not. The two case types, rimmed and rimmed less, headspace completely different. The revolver case headspaces on the rim, limiting how deep into the cylinder chamber the case can travel. The auto pistol headspaces on that tiny case mouth. The shorter .40 case (in a 10mm chamber) is limited only by the extractor hook. Not only can one stress the hook , but stacking tolerances might lead to failures to fire as well.