As said above, the reviews point to it being a considerable amount stronger than LX, which was to the most part, as strong as chocolate (brakes being the exception in my opinion). 6 months after I bought my LX kitted bike, many parts had worn out/broken, whereas I know plenty of riders who have been running SLX for a similar amount of time and riding much harder than I was back then and the kit is still going strong.
I have specc’ed SLX where possible on my current build.