My observations say the correlation is pretty much the other way around: You work more hours because you can't get the stuff done during your allotted time. You're not learning new stuff, you're doing the same you've been doing the previous 8 hours of that: Struggle.
Of course there is brilliant people that just enjoy their work and stay longer every day; but staying longer is not the reason why they're brilliant. Being brilliant, specially on a trade that has a strong creative component, it's not something you can always train. Or at least not by repeating the same tasks over and over.
I see your point and I agree that side projects could help increasing your skills and/or knowledge. However, I'm not sure they're an effective way of doing that or if we [tend to use mostly the skills we have / tend to learn skills that are so different than our daily jobs that can't be applied or aren't useful for professional development]
I'm sure most people here can attest to the anecdotal proof of having a difficult problem and leaving it. Then it suddenly coming to you some point later during non work. This also applies to developing skills. Not just problem solving.