This is my opinion and is based solely on my experience after 15 years of being in the industry, including one successful startup built by me and a friend of mine with a decent "exit" sold to Telecom Italia, and a successful open source software (Redis). Additionally I used to work in different places in the past with many programmers, so what I say is also based on how the success of programmers I know first hand correlates with the amount of work they do.
I also follow two companies here in Sicily that I built in the past, one has six programmers.
So all the above is my "data source", that is not statistically significant.
I don't really think a programmer working 8 hours is going to be less successful of one working 12 hours per day: it is either good enough to be outstanding with the 8 hours, or will fail even with the 12 hours, since the productivity gained by experience, skills, understanding of the problem is the kind of 10x or 100x gain, while from 8 to 12 hours the difference is little (and, again, not sustainable for long time IMHO).
Startups are built by programmers, so I think the same applies to them as well, mostly.
In other words I could say that the output of a programmer correlates more strongly with the work it did to learn writing code, than the work it does writing actual code.
Another important thing is that having fun is not lost time if you want to build a startup. Recently startups are a lot more about providing an "experience" to the user, and enriching yourself in your free time can be very useful to provide something that people want.