First the disclaimer: I am doing these calculations off the top of my head and spent about 10 minutes thinking about this so take the information for what you think it is worth.

If there are 40 players remaining then a player with an average chip stack has a 1/40 chance of winning (assuming all players are of equal ability). A player with an average chip stack also has a 9/40 chance of making a 9 seat final table (again assuming equal ability).

If you have 3 times the average stack you have 3 times the probability of winning 3/40 and you have 27/40 of the average stack at the final table so a 27/40 chance of making the final table.

If you take on the big stack with a 70-30 advantage you double up 70% of the time and have nothing 30% of the time. When you double up, you have 6 times the average stack for a 6/40 chance of winning the tournament and you exceed the average stack for the final table so if you ignore blinds and antes (I know this is unreasonable but it simpliflies things) you now have a 100% chance of making the final table.

Since you beat the chip leader 70% of the time, your chances of winning the tournament increase from 3/40 to .7*6/40 or 4.2/40. Your chances of making the final table drop from 70.5% (27/40) to a maximum of 70% (because you are really never 100% guaranteed to make the final table).

Again, keep in mind I did this on the fly. Also, try posting it in the probability forum those guys are really good at these types of questions.

Dave