Is there any difference in terms of typing performance between using a normal or an ergonomic keyboard?
Ergonomic keyboards are designed for minimizing your risk for repetitive stress injury and carpal tunnel syndrome. As a result, by using such a keyboard you can touch type for more hours without worrying about wrist or finger inflammation. Of course, applying the whole spectrum of ergonomics at the office is what will ensure you stave off any office-related injuries. An ergonomic keyboard is not a cure-all.
It’s true - an ergonomic keyboard affects body positioning; it is not designed to improve typing performance other than comfort.
If you end up deciding to go with a regular keyboard, I suggest keeping it flat. Many detached keyboards have extendable legs, but it actually makes more ergonomic sense to keep keyboards flat against the surface of the desk. For proper hand placement, the wrist should remain neutral, bent slightly upwards. Having the keyboard tipped up may cause you to angle your wrists too harshly. Repeated misuse of the keyboard may lead to long term discomfort and should be avoided.