I don't personally own a .404, (although I wish I did.) I do have a .416 and I would absolutely say that, even though it is a bit heavy, a 400 grain Woodleigh Weldcore (the RN not the Protected Point) out of that gun at 2200-2300fps will work great on plains game and also on anything up to buff. If you wanted a purely plains game bullet, Woodleigh makes a 347 grain Weldcore RN that you could probably get 2500fps out of easily. Thats a solid 250 yard rifle even with the 400 grain bullet. Theorectically, the 347 grain is only dropping 11" at 250 yards and the 400 would only be about 15" at the same distance. Thats pretty flat shooting for a round that will hit harder at 250 yards than a 7x57 does at the muzzle. And lets face it... you don't need 2,000ft.lbs to kill an animal. Airgunners have been proving that for some time now. I think that any loads you have in your .404 will do everything you need them to. Heck, if all you had were 400 grain solids, I wouldn't hesitate to use those! people underestimate the effect that a 1 oz. .40+" bullet traveling at Mach 2 has if it penetrates something vital. A kudu/impala/gemsbok/wildebeest/insert-your-favorite-plains-game-animal-here, will not run far if punched through with a solid from a large-medium bore! If you can get close enough to get them with your black powder rifle, (which I know you can because I pour over and over your 1880's hunt segments, KEEP THEM COMING!) then you have nothing to worry about! Best of luck Gert!
I would also add, That even with a slow (1000fps) 375 grain minie bullet from my muzzleloader, I get meat damage on some smaller animals... It's just something I've learned to accept from killing animals with blunt shaped projectiles hole-punching their way through animals. I've even seen bloodshot meat from bow and arrow kills sometimes.