Nesika Bay is the only one who uses a Bolt/Handle that is made in one piece that I know of. Can't possibly break that baby off! I've got the HD bolt mod job on my rifle, and love the looks and feel. They will also ceramic coat your bolt, and I'll say this much, it slides sooo well, with very little resistance. I may have another rifle done in time. Very fast return, and my rifles are left handed, so there was no problem with that. And yes, they use a very high silver content silver solder to attach bolt handles.
The bolt can be re-tempered after welding, but the shrinkage? Metal contracts when welded, and I would think that the bolt may need to be re aligned after welding. Ever watch a weld "pull"? Ever have to make it "pull" back? I have, and it can be done, but metal does "contract" after welding. I imagine that if it was done prior to "trueing" it might be ok.