My experience with stretching is that it's a very foot specific business, and that a shoe/boot may stretch laterally to become more comfortable, but never length wise. I have a wide foot, so boots that don't come in wide sizes tend to be a bit narrow, but CXL/Shell will stretch enough so that they become nice fitting, and it doesn't matter if the sides of my foot are touching the sides of the boot, as long as the pressure isn't too much. This is preferable than shoes that are 100% comfortable at first, but then stretch and now are too big. For me, 9.5D's start comfortable, and 9D's start uncomfortable. My 744 9's are now absolutely perfect, and the length in the toe isn't too bad. The 9.5D's I suspect, would be too big.
If you have a long, narrow foot, though, you'll find that a snug fit will only come when you don't have enough room in your toes, and that's always going to be uncomfortable. The amount they stretch there will still always result in your toes touching the ends of the boots, which is a Bad Thing.
Also, re: the question of changing the heel with a half-sole, my half-sole is about 4mm, and I didn't change my heel. Bear in mind that 1K's already come with a pretty significant heel compared to dress shoes, so it might be completely unnecessary.