I certainly found it easier to use my knowledge of literary Welsh to help with my speaking ability as I talked to more and more people than is sometimes implied has to be the case. However, firstly - I did find Ssiw helped and speeded that process up tremendously, it involved talking to a lot of people. And secondly - I do know a lot of people who are actually put off by it. I didn’t mind chopping and changing, many people want to jump straight in with something as colloquial as possible, and can be put off if it isn’t. Though as I day I didn’t mind, I can understand that.
But yes, I did find an understanding of the grammar to help me speaking with people (again, that was with speaking to a lot of people, and found ssiw accelerated the procesd again) - and again though, I did find people can stomach differing amounts of that stuff!
Just my experience.