Of all the bigger plains animals in Africa the kudu is the easiest to kill. They don't have the will to live that most of the other equal size game has. Rmemeber to that the Kudu is a brush inhabitant not an open grassland animal. Most animals that live in the open and get shot there will make every effort to get into the thickest bush they can find. Since Kudu are already in that level of Bush they may seem to be less "tough" but are already where they want to be. Killed in the open( where I don't know) they too would likely seem tough and run a long way to the bush.
See what your PH says. If he's got much experience in the hunting game, he will make a similiar comment that a Kudu is not as tough as waterbuck, wildebeast. zeba or any of the other similiar sized game. That's not to say a poor shot will kill one, but a perfect shot will usually crumple them or limit their travel.
Namiba is not like hunting "Africa" as it's wide open desert country with rolling sandy hills of grass and limited brush habitat. Hunting Namibia is more like hunting Saskatchewan, eastern Wyoming, Eastern Colorado etc. Where the bushveld is very think brush like Texas hill country or Mainland Alaska. Short shots are the norm there.
If I were leaving for Namiba I would likely choose a 300 weatherby or 300HH magnum as a first choice. After that the 338, 30/06 etc would be OK. I don't believe in using sub .308 diameter bullets in Africa becasue they don't give a blood trail on African game or in herd conditions. Shoot a 300 weatherby with 200 grain bullets and you will have an easy 350 yards to hit the game easily. With a 338 I would likely shoot a 250 grain or less bullet and zero for 300 yards. Namiba can have long shooting in the wide open country.
There are some areas with Bush and maybe a bit closer shooting. Best you check with your outfitter and PH before you do what I'm telling you. They know their area and habitat and will suggest the right tool for the job!