The 250 grain Hornady HP is a very soft bullet and will do the job on deer size game if you do not have to break any big bones. The 250 grain Nosler SJHP is also a good bet for general deer hunting.
Personally I have had good luck with cast bullets and the .45 Colt. I just adjust the hardness when I load hotter. Do not go much harder than 15 to 20 Brinnell range. I have broken both front shoulders of deer at 50 yards with a 310 grain Keith style slug at around 1200 FPS at around a hardness of 16/18. The same load will give you pass thru shots on the chest of almost any game animal you can name, given a reasonable distance.
There is really not much need to "magnumize" the .45 Colt even though you can. Use its ability to throw a big, heavy slug instead.