BBB - Agree with you on the need to be very skilled to be able to know how the market would react even if you did have inside information.
I don't agree about derivatives though. Options, for instance, are one instrument that are highly sensitive to large moves in their underlying.
But overall, I think the market does a good job of factoring inside info into a price LONG before punters on-the-street get wind of it.
That's why it's still a good idea to consider "Buying the rumour - and selling the news".