Another good data point is the Turbo Timber from EFlite. I've flown it with and without slats a ton. There is a marked improvement with the slats installed. Less wing rock at high(er) Alpha, softer and slower stall, better roll control at slow speed. There are some negative effects as well. Slower roll rate at high speed and inverted performance suffers as expected. I was more interested in the improvements for STOL performance, so the slats are an all win for me. Anyone who claims they don't do anything, at least on the Timber is probably not a good enough pilot to tell the difference.