Does a developer's job affect their performance on certain tasks?

You were correct in your own assumptions:

• Job Title is there for a reason! That person is good at a specific task, and should be used as such. However, the same worker should never be used on two games in a row.
• Choosing a worker with a high stat for a task he wasn't meant to do (IE: Choosing Sound Manager for Writing) is never worth it. It's not possible to get enough stats to make up for the deficit. The worker will be unfocused and perform poorly.
• Hackers should never be left as Hackers. Buy a Career Change Manual from the salesman and give them a role based job. Yes their stats will drop slightly, but it will still be worth it.

One of the best guides I found for the game while I was playing it is this one. Most of these points I learned from that site, and they cover a lot more as well.