Lee's data comes from Hornady, mostly. I never worry about conservative loads myself.
I started shooting cast bullets in a 45 Dutch Beaumont rifle but really got serious shooting them in handguns. Like you, I have fired X thousands of alloy bullets in handguns. For the last 20 years or so, I have been shooting them in rifles with great satisfaction. Lee's views on velocity VS pressure are not his own. Well; to him is might have seemed like an epiphany, but it has been common knowledge in The Cast Bullet Association that cast bullets will hold up quite well when the pressure is matched to the alloy. I have shot 55 grain bullets from a 22-250 at 2800 fps with just slightly over 1.2 MOA accuracy. Not bench rest, but the varmints didn't know the difference. In my other calibers, I have gotten good results. I regularly hunt deer with Brno 308 Winchester using a 188 grain bullet at 2450 fps. It will drop deer like Thor's hammer while delivering sub-MOA groups.
Lee (through Midway, USA) once offered a limited edition 8mm "Karabiner" mold weighing 200 grains and of a gas check design. I have two of these and would be happy to send you one to use in your 8mm. I would slug the bore of your JR and see what the diameter is. I have a feeling this Karabiner will run around .325" but I've found that "J" bore rifles will accept a .322" bullet in the neck and still allow adequate case-neck expansion on firing. If you order a Lee "push through" sizer die in .322" you'd be set. (But for a stick of lube) If you don't like it, you can mail it to JAYB in Southerland and I'll pick it up next time I'm over. Otherwise, you are free to keep it. I have better than 100 molds as it is, ranging from .20 to .72 caliber.
There are some tricks to making cast bullets work in rifles. I know 99% of them and would be happy to help you get some Pb slung out of your rifle. There is nothing like shooting a small group with a bullet you made and fit to the rifle yourself. Very satisfying, indeed.** ~Muir
** And yes, very economical.