The 787-800 is a truer replacement for the 757-200 than the 737-900
The 787-900 will be a replacement for the 767-300
The 787-1000 will be a replacement for the 747-400.
The performance numbers don't match up exactly, but the 787 series will outperform their respective 757/767/747 series counterpart.
Take a look at the 757-200 and how it was being used a lot in the past few years by US Airlines to do 'thinner' international routes. The 787-800 can do those and hold about 70 more folks while burning less fuel (I think, @
Minuteman you have the numbers?) than the 757-200.