A couple of things I might suggest to you... Remember 100 grains of T7 pellets are not like a 100 grains of loose T7 or BH. Even a 150 grains of T7 pellets is not really a 150 grains of T7 more like 120 grains of the more energetic powders like Swiss BP, loose T7 and BH.

T7 pellets are manufactured to provide approximately the same energy as regular Black Powder or any of the Pyro powders. So theoretically your 100 grain pellet load is approxmately like 85 grains of T7.

Your Omega might do just fine with 150 grains of pellets, you will not really know until you shoot it. I normally shoot 110 grains of loose T7-2f from my Omega and have used 120 grains of loose T7 with a 300 grain Nosler Partition for elk. In most cases this is probably more than you might need but in the instance of an angry bull @ 100+ yards the extra may be needed, especially with just one shot.

In all honesty the 250 might work just fine in a lot of instances, especially shooting cows but again for instances where more might be better - I would feel better with a heavier bullet and at the same time a stronger bullet like the Barnes mentioned or a Lehigh/Bloodline. If Nosler were still making or you could find some .458-300 grain partitions they would be excellent also also. One thing I might suggest, if you were to decide to stay with a normal copper/lead bullet you might also look at a .458 bullet (45-70), they are built to perform from a rifle.

Kinda just thinking out loud... or in black and white