50% mature SD culture and it didn't dissolve the dough into mush? I've seen it happen at 35% let alone 50%. Wow.
Craig, I use a young active starter and not a mature active starter. If you make a starter or leaven starting with a 10% seed at 100% hydration, it may be ready in 6+ hours to use. I use it just as it is doming and not on the way down. I taste it to make sure its not too acidic. At this point it behaves just like any other starter but will ferment the dough quicker (the more you use that is). If you are going to use a 50% plus starter amount, you have to account for the huge jump in hydration. For example, a dough that is 63% hydration, a 50% starter (of the flour amount) will increase the hydration by 7%, so you end up with a 70% total hydration, so you will have to adjust your original hydration down to end up at your desired hydration level. The starter behaves the same just quicker or slower.
But a word of caution to anyone wanting to use a high starter or leaven amount. You can't simply pull an old starter from the fridge with hooch, mix it back in and use it. If you do, this is where the acids will ruin your dough. You need an active healthy young starter. That means dump the hooch and most of the starter (95-99% of the old starter) feed and refeed and get it active at room temps prior to using it. You want to taste it first and make sure it's not too acidic. A little is okay. It should smell sweet or a bit fruity. This constitutes an active young starter. If you pull a starter from the fridge that hasn't been fed within a week, even if it has no hooch, it may have a lot of acid. If you only dump half and refeed only 1 time, once it becomes active, you'll have more of an active mature starter. If you taste the raw starter at this point, it will have quite a bit more bite to it than a young starter. Big difference in the end crumb texture mainly due to the excess acids.
If your starter was fed with a small seed amount 2 days prior and kept in the fridge for just a few days, it can come out of the fridge, be warmed to room temps actively or passively, and be ready to use within 2-3 hours without feeding and have very little acid in it. It just depends on the state of your starter and how you maintain it.