As fine as a reputation as the Whitworths enjoy, it's much harder to develop an accurate load for them than a minie gun. Not only do you have to find a bullet mold, but also the right powder charge (and perhaps a lubricated wad). Some folks do paper patching on them and so it becomes a real labor of love. Member David Minshall who runs the Long Range Rifle Website can tell you more about it.
Besides, even if you could shoot Bambi (yum yum) at long range, remember, most deer are harvested at closer range. Last, if you ever have to reload, you can't do it quickly with a Whitworth (unlike the minie gun).