That's not really quantifiable though, until you've seen him play in your offense.
Let me put it differently
If you run an option offense or something, you're not going to sign an old slow immobile QB as a backup because he's the best guy available
You're gonna get the guy whose skillset best fits what you wanna do.
You're not gonna take the immobile guy and then suddenly completley overhaul your offense to fit his skills
You might change some things a little, but nothing crazy