Basically, you cant just replace a .357 barrel for a .257 barrel on a big bore due to the air flow path and expect it to perform by shooting the .257 that much faster.
You certainly can shoot slugs at supersonic, however as with all air rifles, they are nit powerful enough to shoot high enough above the transonic point to remain supersonic to any great distance. Stability and therefore accuracy will be impacted (lost). If you read through airgun threads on slugs, you will note they are all subsonic, and even when we increase power (valves, pressures, etc) we dont strive to shoot supersonic with a certain weight, we shoot heavier longer projectiles higher SD to capture the higher BC. This is also why it may be important to consider barrel twist rate and the desired weight/length of your projectile. It is possinle to shoot slugs from choked barrels, however you will really need to pay attention diameters.
In general, a cast bullet should travel faster than a pellet to take advantages of its ballistics, however you may still have great success in accuracy at slower speeds. Some it depends on what you want. In some calibers and platforms, that is the case. For example my first .457 was a Sam Yang and did not have near the power capbility of any of my Texans, but was quite accurate and good energy to my 75yd limit at the time. One of my .308 had a fast twist rate and did not like shorter bullets, so the heaviers went sub 900fps at the pressure I was tuning at and was quite accurate for my 100-120yd purpose. I increased pressure for longer distances.
If shot count is a concern, most big bore slug shooters do not have high capacities (certainly some better than others, as well as potential for improvements through tuning and mods). My .308 rifles would get a handful of shots, however my Rainstorm .30 shooting NSA was more efficient and also a repeater, if that is something else you desire. Power is different (which again is why you can just change out a barrel in most air rilfes designed to shoot pellets) where my .308s could go over 200fpe my Rainstorm .30 with NSA slugs peaked around 110fpe. But with a quick tune, my Rainstorm could go back to shooting pellets at 75fpe for 14+ shots.