I wonder what length barrel the .275 140gr load was chronographed in. Probably 28" which is usual par for the course for the English when chronographing their ammunition. With the Rigby No 2 rifle having a 23.5" barrel Rigby were stretching the truth somewhat with their advertised MV of 3000fps.
I can't imagine them loading to much over the lower pressure level that the 7x57/.275 would have been loaded back in those days.

It is not possible or easy to get the 7mm-08 in modern loadings up to a MV of 3000fps with a 140gr bullet in a 24" barrel (Hornady Superformance gets to 2950fps with a 139gr bullet and their special powder - 2900fps in a 24" barrel I have chronographed) and while the 7x57 cartridge has a little more case capacity, powders and pressure specifications of the day wouldn't have seen the .275 cartridge for the Rigby No.2 approaching anywhere near 3000fps MV.
Nevertheless I imagine the lighter bullet at somewhere round 2800fps MV, maybe optimistic at that, would have been a better performer on smaller game than the commonly loaded slow heavy 175gr bullet.