How fast you can drive them depends on which gun your shooting and if it has a gas check.
Copper gas check will prevent the bullet from failing and leading up your rifling.
These bullet barn bullets are quite hard but they work nicely i've been using them for nearly 10 years or so without issue , I settled on gas check 405 Grains awhile back and bought like 800 rounds so Im still using them up.
I don't shoot my 45-70 as much anymore so they are lasting quite some time it seems.
Anyhow as far as how hot is too hot.
If your shooting a Marlin Leaver action Id say 405Grn at 1800FPS is your upper limits of the gun I would go no further and even some may say that is too dang much already.
If your shooting a trapdoor or anything like that reduce loads accordingly.
You need a good reloading handbook they will tell you which actions can take what pressures and follow accordingly.
The 45-70Gov is one rifle that i've found doesn't always shoot better at higher pressures like some cartridges prefer.
All it does is beat the heck out of your shoulder and make you shoot worse

.
So keep things reasonable and work up a nice load and take your time and things should work out for you.