Again, I will parrot Bonanza---

Agreed, but what Double guns would be possible for lighter weight?

Acording to my balistic tables the 45-70 max loaded goes 3000 fpe so that comes far short of the goal of 4000 fpe.My previous choice of 45-70 obvioously falls way short.

A lightweight 450 No 2? Some other 450 Double for 4000 fpe?
You should be able shoot a 458 cal 450 gr bullet at 2000 fps out of a 9 pound gun without getting beat up too bad--
This should handle any close range NA hunting situation easily.The problem is that you couldn't upgrade to full African performance/ballistics without getting the snot beat out of you from a 9 pound gun.

A QD scoped 450/400 SS polymer stocked would be perfect( and yes-very ugly!) and more versatile but I bet shooting 458 bore bullets in a Double would really turn on American hunters!! Sadly it may not be feasible to do this and thus a smaller lighter weight Double caliber may be needed to work in the states as well as other hunting locals.