One of the major symptoms of my stroke was seriously truncated working memory, and I spent months both training it back and learning to work around the limitations of it.

So i agree that there are strategies that can overcome the limits of working memory,though I wouldn't describe them as "thinking more"... it was more like saving state externally on a regular basis, and developing useful habits of interacting with that saved state. More generally, it's not a question of doing the same thing for longer, it's a question of doing different things that end up taking longer. It's "thinking differently, for longer."

That being said, though, I cannot begin to describe how much smarter I felt (and seemed) when the damage began to heal and I could start doing stuff in my head again.

In general, the limits imposed by working memory can be overcome by the use of more time-consuming strategies. Therefore, although performance might improve if working memory were larger, it might also improve if subjects simply thought more.

Baron, Thinking and Deciding