Ahhh, ballistics. I'll jump into a ballistics conversation almost every time, even if I've made identical claims 50 times before!
First, bismuth doesn't retain downrange energy as well as lead or copper plated lead (as suggested above). It's less dense. It's superior to steel, but also worse in terms of retained energy than TSS, tungsten-matrix, tungsten-iron, & true HeviShot. (HeviSteel is just steel. Nothing heavy about it.)
Also, although obviously shot can be flung so fast as to significantly increase energy at a particular range, typically the required increase in velocity is great (usually 300 fps or more) & only serves to make decent patterns less likely, while also greatly increasing cost, recoil, & maybe wear & tear on your gun. Yes, things like nickel plated shot, funky wads, lengthened forcing cones, & finding that 1 perfect after market choke tube out of 100 available can be used to TRY to achieve a good pattern, but it's just plain less likely with high speed loads. Why make a good pattern harder to attain?
Let's look at some pheasant shot sizes & velocities (common on today's market) to see what velocity can & can't do for effective range of a pellet. This is using the KPY Shotshell Ballistics program, set to achieve 1.5" of ballistic gel penetration at a certain range, which "they say" appropriately represents a pheasant. Whether "they" are right or not, doesn't really matter. The comparison is still valid.
Lead 5 @ 1300 fps, 56 yds.
I don't need more effective range than 56 yds, so why bother increasing the velocity of a lead 5?
Lead 6 @ 1300 fps, 45 yds.
Lead 6 @ 1500 fps, 51 yds.
Lead 6 @ 1700 fps, 56 yds.
Bismuth 5 @ 1300 fps, 40 yds.
Bismuth 5 @ 1500 fps, 46 yds.
Bismuth 5 @ 1670 fps, 50 yds.
Bismuth 4 @ 1300 fps, 50 yds.
In summary, muzzle velocity of a lead 6 or bismuth 5 must be increased about 400 fps to achieve the same effective range as simply increasing 1 shot size at the lower velocity & increasing chances of a good pattern & minimizing cost, recoil, & gun wear.
Really the only place for hyper fast loads is with steel, in order to keep payloads & shot sizes within the realm appropriate with modern shotgun design & the birds we hunt. But there's a reason the 3.5" 12 gauge was developed. Steel shot is that reason.