The entry-level Surface book (e.g. the $1,500 version) doesn't come with a dedicated GPU. So I'd think it ought to perform pretty much the same with and without the keyboard. The CPU in the entry-level Book is the same as the mid-level Pro (which is also the one you get with 8GB RAM), making that probably the best comparison.From the website description, the GPU is in the keyboard so if you detach it, it performs worse than if it's linked, but since the surface pro doesn't have a GPU to begin with, it's probably about the same.
So, based on specs, I'd say same performance, 3hrs longer battery life (extra battery in the keyboard?), a second USB 3.0 port, and the real keyboard.