For most of my guns, I don't go above 1,500 fps and I use soft cast bullets made out of pure wheelweights with only a smidgen of tin added for a BHN of around 8-13, depending upon how many stick on wheel weights are in the mix. You will not break up a soft cast bullet. Like has already been said, that happens with hard cast and I hate hard cast bullets.
I've no experience with plain base soft cast bullets between 1,600 and 1,700 fps. At the higher velocities, say above 1,600 or 1,700 fps, I'd recommend a gas check. I use one for my 30-30 cast bullets and put them out the barrel at around 2,100 fps with no leading I've ever been able to detect. They don't break up either, at least not in the two Whitetail bucks I've taken with that bullet at 60 yards and 80 yards respectively.
I'd differ a bit from what cowboykell said about not being able to bump of cast bullets with smokeless. It can be done, I've done it plenty, and I've relied on it for a few of my old guns which had a groove diameter larger than what I could chamber. The secret is to use soft cast bullets and a load that gives you the same pressure curve as black powder. Black gives a nice sharp spike, which is why it can bump up soft cast bullets. Smokeless powders come in a variety of burn rates, some faster and some slower than FFFg and FFg. The crossover point is somewhere around Blue Dot/2400 FOR THE SAME BULLET WEIGHT AND VELOCITY. Slower powders will give you a lower pressure spike than BP and faster powders will give you a higher pressure spike than BP. That is why so many people get such nice accuracy using Unique. It gives a massive pressure spike (compared to BP) and will bump up that soft cast bullet nicely to fill the bore. I cannot speak for hard cast bullets, and I doubt you can bump them up, without excessive pressure spikes. If you buy your cast bullets, chance are almost certain that they are hardcast bullets. Hard cast bullets are particularly hard on revolvers, especially older ones. When that hard cast (BHN higher than 20, which is ridiculous) hits the forcing cone, if it is oversize at all it gives a massive impact. Over time, you will stretch your frame if you are shooting a 1st gen Colt SAA or an early S&W top break. Furthermore, hardcast bullets are pathetic at expanding, whereas the soft cast ones will sometimes expand if they hit fast enough. Now I cannot speak for tough dangerous game; maybe you want a harder cast bullet for those, I just have zero experience there, but 86er would know and if I recall, I think he does recommend harder cast (BHN ?) bullets for tough dangerous game, but don't count on my recollection of what he has written, get it straight from him.
Here are a few curves comparing fast and slow smokeless and BP. It is not all apples to apples so you'll have to extrapolate, but you get a good idea of the comparisons. By the way, you can see from the curves why Trailboss (a very fast powder around the speed of Bullseye) seems to do well as far as accuracy goes; it has a ridiculous pressure spike for the same bullet and velocity, much higher than BP. With a peak pressure like that, your soft cast lead bullet is going to flow like plastic into the bore, bumping up totally and completely. I stay away from Trail Boss in original BP guns.
You can also see why I like SR4759 as a good powder for BP cartridges. For the same bullet weight and velocity, it has a very close pressure curve to BP (in the curves below, it is a little flatter because of the lighter bullet. You stick a 500 grain bullet and the curves will be very close.) 2400 is even better with pretty much an identical pressure curve to BP (too bad they didn't include a plot for 2400 in that set).
Recovering paper patched and regular greasegroove cast bullets reveal lots of things that makes a feller say hmmmmmm 
