Marius,
Have no fear hunting a good client with the 6.5 Swede shooting 140's. It's effectness seems out of proportion to the paper ballistics. You won't see a difference between the Swede and the .280. I've had great success in RSA with the Swede.
I wouldn't use if for eland, and I have witnessed gemsbok soak up the lead and move on, but they use that round in Scandanavia to take trucks loads of moose every year.
I've had quite a few hunters bring the 6.5, and they do work great. I've had a lady take a Kudu with a 25.06.
I still prefer my hunters to pack a little more than 6.5mm on something like. Kudu,Gemsbuck or Wildebeest.
Like i said, can it be done, absolutely, but I see many calibers listed by the OP that will pack a little more heat than the 6.5
