I left mine at factory setting. My efficiency is generally around 80% going by my BrewTarget software.
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Here's a good article/experiment pitting rehydrated vs dry/sprinkled yeast. My takehome from it is that the effects on fermentation and taste vary from strain to strain, but the differences don't seem to be too significant.
In the end, I personally think of it as the decision whether to rehydrate or not equates most closely to pitching rate. I've always been a rehydrator, but I think I'm going to switch to sprinkling dry for lower gravity beers. I will probably continue to rehydrate for beers in the 1.060 and up range, especially in beers where I want a clean fermentation (although I'd probably still sprinkle something like T-58 if I'm looking for more esters).