Here's my problem with people who put in 80 hours a week: they don't write good code. At my first job - way back in the last millenium - we were creating a carrier class network device. I was hired to run the mail servers but ended up building an embedded linux that ran the PowerPC on our hardware - we also had a Strong ARM that ran a gig ethernet but that was under another guy. So, how did I go from mail server to an integral piece of the product? I took over for the guy who put a lot of time into not solving a problem. Before you can even get linux running on your hardware, you need a boot loader to initialize your hardware and get it into a state where linux will run. Our "hard worker" volunteered to write a boot loader and spent 60+ hours a week for the next 6 months writing it. But it never worked.
Management decided to let me take a crack at the boot loader. Now, I was in my early 20's. I had bars to drink at, a girlfriend to get some lovin' from, and parties to go to. I didn't want to spend 60 hours a week at my desk. So, I did what any lazy hacker would do: I found an open source project that was close to what we needed. The project I started from was PPCBoot and it was started by Wolfgang Denk for the purpose of booting hardware running a Power PC processor. I spent 2 weeks telling everyone else in the company that I needed their help on X, Y or Z; getting them to write some code; and burning the new version of PPCBoot onto the flash chip. After 2 40 hour weeks, something amazing happened: it worked. It configured the hardware and handed control to linux which booted up.
Anyway, that's my reason not to spend more than 40 hours at work in a week. All of the people who I've seen put in all those hours aren't really working. They're just playing with a neat project because they've always wanted to write a boot loader. The purpose of you job isn't to write code, it's to ship a product. If you keep that in the front of your mind at all times and focus your efforts on shipping product, you can "work" 40 hours a week and code a pet project on your own time.