I've been hunting the .45-70 for over 20yrs, it's a bit of a guilty pleasure for me, which goes against all of my typical preferences. I've loaded everything from 200grn bullets up to 500's, from plinking pressures up to irresponsibly high pressures in 1895's and No.1's. I'm about 8-9yrs into the 325grn FTX bullet in the 45-70.
The Hornady Leverevolution factory loads with the 325grn FTX run 1795 average in our 3 Guide Guns (2 + 1 SBL, same rifle, different stock). In 2011, I threw one 250yrds into the buck pictured below, and recovered the bullet, also pictured, under the hide on the far side. Gutted, skinned, and with his head cut off at the shoulders, he was over 190lbs hanging weight at my processor, weighing the head and adding it to their hanging weight, the processor told me he would have gone 315-325 on the hoof. The unfortunate circumstance of 1795fps and a low BC bullet is the resulting 30" of hold over I had to manage, but I stuck it right through his heart, shredding the near lung into jello, same for the heart, and cutting his far lung in half. Not a small whitetail, not a short shot for the 45-70, not a high velocity, and it didn't hit anything hard on the way through.
The FTX bullet really wants to open, so if you're hitting deer up close with it, it's going to shed its tip and lose a lot of weight, but I've always gotten pass throughs on deer under 150yrds with them, even at "Marlin Only" velocities - but the bullet isn't pretty, and I don't hit the shoulder with them. I'm positive it'll crush the shoulder and anchor the deer even if I punch the nearside shoulder, but I do not trust the bullet to hold together well enough to kill the deer quickly if I try to run it through a big deer's shoulder joint first.
The extra speed does make a bit of a difference. I've gotten that FTX up over 2100fps in the Guide Gun's short barrel, up from 1795 factory level. It saves me ~10" of drop at 250yrds, which for me, means my "manageable range" improves from a max of 300yrds, out to 380yrds, with a comfortable range improving from 250 where things get a little hairy, out to 300yrds. Adding 50-80yrds doesn't seem like a lot, especially considering how much harder the rifle recoils, but when we're talking about 250 vs. 300yrds, it's worth it on some of my properties where a little longer shot might be common.
I'm not a huge fan of the 325FTX, since it IS so lightly constructed, but I do stick with it because it does shoot so well in my rifles. I've replaced the 3-9x on top which was used to shoot the group below, which resolved some of my vertical dispersion, but getting that bullet to shoot slightly Sub to 1MOA isn't a chore, anywhere along the spectrum of velocities I've described.
The Hammer of Thor is an awesome force, and it gets thrown around a lot about heavy bullet rifles, but there's a distinct reality - when you're playing whitetail tag with a 45-70, your legs don't get tired, which I can't say is nearly as reliable even with a 7RM or 30-06. Would a .45-70 be my ONLY deer rifle? Nope. Do I take it out almost every year? Yup.