I don't see how it would cut the mixture, but it would dilute it some based on how much gas/oil you have in your tank. So, if you want to add the stuff and you're concerned about dilution, just add additional oil to bring the mix back to the proper ratio.
As for cleaning the carb in your outboard...I dunno. Do you have a carb related engine problem now? If you do, the only way to truely fix it is to purchase a carb rebuild kit. Then take the carb apart and clean every little passageway with carb cleaner. After that run compressed air through them and make sure they're all open! Carefully reassemble the carb. Fixes lots of problems, especially in engines that have been stored for extended periods.