Can I use the same powder and amount for a jacketed bullet as the same size diameter hard cast bullet?
No, you cannot if using the same size diameter cast as a jacketed bullet. A jacketed bullet does not have to seal the gasses behind the bullet to prevent gas cutting as does a cast bullet whether it be hard or soft. Change the cast bullet diameter to .002-.003 over bore diameter and the answer becomes yes, you can provided you pick one of the slower burning powders in the data to load and start with the starting load.
Will the bullet composition significantly change the amounts of pressure?
Pressure will normally be less with a cast bullet than a jacketed bullet. All things being equal you can get the same jacketed bullet speed with a cast bullet with less powder and less pressure until you get up to and above the 2700 fps velocity range.
In my opinion the term hard cast has become over blown and confusing to most new reloaders. Today hard cast means something in the 21 BHN or above range. In Elmer Keiths days anything above 11 BHN was considered hard cast. Whether it be hard cast or soft cast the cast bullet must fit the bore properly or leading will occur. I see a lot more leading problems with hard cast than I do with softer cast so why bother with alloy that is brittle and easily shatters on impact.
Yesterday I was shooting a H&R 357 mag rifle which has a very generous bore. So generous in fact I take 357 maxi brass and cut it down to where it is between 357 maxi and Dan Wesson 360 brass and it shoots fine. With a load of 19 grains of Lil'Gun behind a 192 grain cast gas checked bullet sized .3585, I put 50 rounds of these down range at 2012 fps. That is right up there with a 30-30 170 grain performance. Accuracy was excellent and not one speck of leading in the barrel. The alloy of the bullets were wheel weights with 2% tin at 12 BHN.
Now having said all that and not knowing if you cast your own or not and I suspect not, then you are kind of at a disadvantage in buying cast that will fit your guns bore. I have in the past bought 180 grain casts from "Cast Performance" for the 357 mag rifle. If memory serves me right their casts are somewhere around 20 BHN and the ones I ordered were sized at .358 diameter. These shot very well for me. So if you are buying your casts, I would shop around for venders that sell different sized casts for their calibers. Cast Performance is the only vender I have bought from but I'm sure there are others that sell to the size you need.