Just my opinion... If I hunt eland again, I will use a 375H&H, again, or a 338 or 35 Whelan. Ive only hunted them in quite open country, 1st time the wildebeest nearby were on alert and the eland shot was about 175yds slightly away through the heart. The second time a similar distance but it took off, my second shot dropped it, and the 375 with grand slam's was perfect. Both times the bull Eland were massive, almost the same weight and possibly heavier in the chest than a cape buffalo. I'm sure a 300WM (etc) and tough 180 grainer is great for most shots on Eland, and if we are talking Lord Derby with a bigger price tag then I would prefer use a 340WM or 9.3x62, or 375H&H, because with a heavy bullet there are more chances to take the shot, with brush and angle factors.. and a long way to fly back again.
This thread started about the 6.5x55, and yes I would confidently use it on small to medium game to 200 yards, and even upto Kudu or Waterbuck at 150yds. However I personally would prefer a 3006 and 180 pill for Zebra or Wildebeest... and for the Kudu or Waterbuck.... and a 300mag if I wanted to go 200-300yds.
A long time ago a lot of PHs trembled when visitors arrived with 270Weatherby Magnums and 264WMs.. either a good hit side on on medium game, or a disaster... and long shots would have had a few nightmares. Luckily people stuck to 300s, 338s, 9.3x62mm for the tougher game. But everything goes in cycles, and we are on the smaller, faster and foot pound cycle again, to sell more guns in USA, so we think we can shoot further with less recoil.?? Luckily we have fantastic bullets available now... as a bullet that what drops a deer or blesbuck at 400, may not drop a wildebeest or zebra at 400.
My resposibility as a shooter is to kill with one shot, however things will occassionally go wrong and shots are sometimes off a bit, and that second shot will actually become the kill shot, and will have to be aimed, taken faster and better placed, and 99% chance its a raking shot. Thats where a 180 grainer from 06 or 300 comes into being. Not many African PHs are going to relax much when a 6.5 Super boomer, monster scope and muzzle brake gets layed on the sand for a 490yd shot. If the shot wounds, its a long way to catch up, and a long day ahead.