I have used the DS, the K10D and the K20D with much joy and appreciation in shooting pleasure, but never honestly saw much difference in the AF abilities between any of them. For most of my needs, it mattered little if the AF was slow, but when it did matter, I was usually disappointed.
I didn't really expect much more from the K5, so it was a huge surprise to find that it is indeed a major improvement over its predecessors.
An example I often use is my 50-500 Bigma, a big and slow focusing lens that likes to hunt like a hound dog. Manual is often the best way to use it.....but with the K5 it is probably a good 60% faster and hunting is not frequent, even in lower lighting conditions. My other fast lenses are .....Fast!
You will notice a huge difference over your GX-10, I don't have the least doubt of it.
Best Regards