Hi folks, not new to GB's but new to this forum.
While I have never had a load close to what Gemsbuck posted, I did had a 400gr load closer to the 2100fps range from my BC. You have to remember when comparing the listed loads to the BC, that the Marlin and Ruger, most likely have 20 or 22" barrels, while the Buffalo Classic has a 32" barrel. That can make a considerable difference in speed. Not sure if it would make up for 400fps though, I doubt it personally.
I have not, but have read of others easily reaching 2400fps with a 300gr bullet from their Handirifle or BC's, but it is possible. That is a very stout load though.
In my opinion, since Kongoni was specific about 100yds, and plains game, seems to me the 45-70 would be well within its element. I have never shot, but only read of the Rigby. I have watched a guy at my range shoot his Ruger #1 in the Rigby but I never fired it. It seems to have WAAY more potential than the 45-70 for sure, but it doesn't sound to me like Kongoni needs all that power.