I don't think the angle vs straight on a backplate will cause a noticeable meaningful difference.
Some back-plates (some sidemount, freedom-plate) also have the attachment-points in the middle/spine area. The major difference I've noticed playing around with a number of different styles of backplates and harnesses that are radically different, is that how you adjust the harness changes. However, once it's adjusted correctly, you probably won't notice the difference.
It's hard to describe in precise terms, partially because of the number of various "weird" sidemount harness/backplate/etc systems I've tried (and a couple backplate ones). However, to provide an example, some setups work a little better if it sits lower on my back. I've also noticed for those systems that sit lower, a chest-strap is a little more useful. The crotch-strap also sometimes has to be adjusted a little different too (also hard to describe).