The way that most CPSA comps are now set, you get shoot pretty much any cartridge for 90-95% of the targets (as long you you put the gun in the right place!) There will be a few targets that are either very close, very fast or 60+ yards but not many. I now use Wiltshire Rod & Gun branded 28 7.5s shells (Eley in disguise) for everything, apart from long stuff when I use Gamebore Dark Gold 6.5s and very close when I use Hull Pro Pistons.
I guess it's all about the confidence that a gun/cartridge.choke gives you and for me, this combination works.