"Would I use it for Cape Buffalo? Yes, if it was my only rifle..... if I had a choice? "

Well our bullet testing team had a choice and killed several cape buff with 45-70 and 45-90 Winchester 1886 rifles.
Our fave for buff was the Alaska Bullet Works Kodiak 450 grain heavy full metal jacket bullet loaded to around 2000 in 45-70 and 2150 fps in 45-90. Most were shoot -throughs.
For the ele, both Punch and North fork bullets shot through the head and into the body.
We used Winchester 1886 rifles as some of our loads were, well "warm" and because we all shoot the 1886.

It is the bullet that kills, so we tried a wide variety and now still use the ones that came out best. I still have some of those DG loads and am having the 45-70 chamber of my DR extended to handle the 45-90 ammo and still be able to shoot the 45-70 (as does my 1886 45-90). No worries about accuracy and such as I have taken a lot of non DG with factory 45-70 ammo.

No worries about the conversion as three makers of double rifles say it is a no-brainer.
It has 26 inch barrels that each shoot sub minute of angle: