The .35 Whelen is a wonderful round. It was in the Whelen's and Keith's day and still is today.
: I'd stick with a good 250 gr. round nose, and practise with it. Learn it's trajectory, how to hold for varous ranges and it will provide dead animals. It just doens't get better than that. Shoot that rifle until it becomes a good friend and you'll not be wanting for a flatter shooting or higher velocity rifle.
: It was an unfortunate time when Hornady dropped the 280RN - what a wonderful .35 cal bullet that was in virtually all the .35 cal cases. Maybe I'm dating myself. Even the .358 Winny would get it moving along at 2,200fps - great moose and elk bullet.
: Speer's 250gr. Spitzer does a credible job up here in Canada, but a 250RN to 300gr. RN would be my choice for Africa. I expect Woodleigh makes superb bullets within that weight range.
: Improving the chamber to the Ackley version will provide somewhat higher velocities with slightly flatter trajectories, but they really aren't needed (or even noticed n the field). A good bullet at a modest 2,500fps just plain works well & the .35 Whelen will provide it.
: Data showing the .338/06 matching or even exceeding the .35 Whelen for speeds with the same weight bullets merely means they've loaded the .338/06 to higher pressures. It's the same case and if loaded to identical pressure, the larger bore will always provide higher velocities than the smaller bore size. That old '06 case will take 'modern' pressures of up to 63,000PSI, but this isn't necessary for good game killing ballistics. Mosest loads in the old standards of velocties in the 2,500fps range do the work without overstressing standard bullet designs. Today, with a flood of good bullets, we can do little better than follow what has worked before.